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1448 lines
52 KiB
1448 lines
52 KiB
.. _kernel_hacking_lock: |
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=========================== |
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Unreliable Guide To Locking |
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=========================== |
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:Author: Rusty Russell |
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Introduction |
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============ |
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Welcome, to Rusty's Remarkably Unreliable Guide to Kernel Locking |
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issues. This document describes the locking systems in the Linux Kernel |
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in 2.6. |
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With the wide availability of HyperThreading, and preemption in the |
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Linux Kernel, everyone hacking on the kernel needs to know the |
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fundamentals of concurrency and locking for SMP. |
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The Problem With Concurrency |
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============================ |
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(Skip this if you know what a Race Condition is). |
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In a normal program, you can increment a counter like so: |
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:: |
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very_important_count++; |
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This is what they would expect to happen: |
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.. table:: Expected Results |
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+------------------------------------+------------------------------------+ |
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| Instance 1 | Instance 2 | |
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+====================================+====================================+ |
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| read very_important_count (5) | | |
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+------------------------------------+------------------------------------+ |
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| add 1 (6) | | |
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+------------------------------------+------------------------------------+ |
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| write very_important_count (6) | | |
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+------------------------------------+------------------------------------+ |
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| | read very_important_count (6) | |
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+------------------------------------+------------------------------------+ |
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| | add 1 (7) | |
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+------------------------------------+------------------------------------+ |
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| | write very_important_count (7) | |
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+------------------------------------+------------------------------------+ |
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This is what might happen: |
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.. table:: Possible Results |
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+------------------------------------+------------------------------------+ |
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| Instance 1 | Instance 2 | |
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+====================================+====================================+ |
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| read very_important_count (5) | | |
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+------------------------------------+------------------------------------+ |
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| | read very_important_count (5) | |
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+------------------------------------+------------------------------------+ |
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| add 1 (6) | | |
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+------------------------------------+------------------------------------+ |
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| | add 1 (6) | |
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+------------------------------------+------------------------------------+ |
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| write very_important_count (6) | | |
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+------------------------------------+------------------------------------+ |
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| | write very_important_count (6) | |
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+------------------------------------+------------------------------------+ |
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Race Conditions and Critical Regions |
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------------------------------------ |
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This overlap, where the result depends on the relative timing of |
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multiple tasks, is called a race condition. The piece of code containing |
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the concurrency issue is called a critical region. And especially since |
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Linux starting running on SMP machines, they became one of the major |
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issues in kernel design and implementation. |
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Preemption can have the same effect, even if there is only one CPU: by |
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preempting one task during the critical region, we have exactly the same |
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race condition. In this case the thread which preempts might run the |
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critical region itself. |
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The solution is to recognize when these simultaneous accesses occur, and |
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use locks to make sure that only one instance can enter the critical |
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region at any time. There are many friendly primitives in the Linux |
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kernel to help you do this. And then there are the unfriendly |
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primitives, but I'll pretend they don't exist. |
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Locking in the Linux Kernel |
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=========================== |
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If I could give you one piece of advice: never sleep with anyone crazier |
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than yourself. But if I had to give you advice on locking: **keep it |
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simple**. |
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Be reluctant to introduce new locks. |
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Strangely enough, this last one is the exact reverse of my advice when |
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you **have** slept with someone crazier than yourself. And you should |
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think about getting a big dog. |
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Two Main Types of Kernel Locks: Spinlocks and Mutexes |
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----------------------------------------------------- |
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There are two main types of kernel locks. The fundamental type is the |
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spinlock (``include/asm/spinlock.h``), which is a very simple |
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single-holder lock: if you can't get the spinlock, you keep trying |
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(spinning) until you can. Spinlocks are very small and fast, and can be |
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used anywhere. |
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The second type is a mutex (``include/linux/mutex.h``): it is like a |
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spinlock, but you may block holding a mutex. If you can't lock a mutex, |
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your task will suspend itself, and be woken up when the mutex is |
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released. This means the CPU can do something else while you are |
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waiting. There are many cases when you simply can't sleep (see |
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`What Functions Are Safe To Call From Interrupts?`_), |
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and so have to use a spinlock instead. |
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Neither type of lock is recursive: see |
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`Deadlock: Simple and Advanced`_. |
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Locks and Uniprocessor Kernels |
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------------------------------ |
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For kernels compiled without ``CONFIG_SMP``, and without |
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``CONFIG_PREEMPT`` spinlocks do not exist at all. This is an excellent |
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design decision: when no-one else can run at the same time, there is no |
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reason to have a lock. |
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If the kernel is compiled without ``CONFIG_SMP``, but ``CONFIG_PREEMPT`` |
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is set, then spinlocks simply disable preemption, which is sufficient to |
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prevent any races. For most purposes, we can think of preemption as |
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equivalent to SMP, and not worry about it separately. |
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You should always test your locking code with ``CONFIG_SMP`` and |
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``CONFIG_PREEMPT`` enabled, even if you don't have an SMP test box, |
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because it will still catch some kinds of locking bugs. |
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Mutexes still exist, because they are required for synchronization |
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between user contexts, as we will see below. |
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Locking Only In User Context |
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---------------------------- |
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If you have a data structure which is only ever accessed from user |
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context, then you can use a simple mutex (``include/linux/mutex.h``) to |
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protect it. This is the most trivial case: you initialize the mutex. |
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Then you can call mutex_lock_interruptible() to grab the |
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mutex, and mutex_unlock() to release it. There is also a |
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mutex_lock(), which should be avoided, because it will |
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not return if a signal is received. |
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Example: ``net/netfilter/nf_sockopt.c`` allows registration of new |
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setsockopt() and getsockopt() calls, with |
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nf_register_sockopt(). Registration and de-registration |
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are only done on module load and unload (and boot time, where there is |
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no concurrency), and the list of registrations is only consulted for an |
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unknown setsockopt() or getsockopt() system |
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call. The ``nf_sockopt_mutex`` is perfect to protect this, especially |
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since the setsockopt and getsockopt calls may well sleep. |
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Locking Between User Context and Softirqs |
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----------------------------------------- |
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If a softirq shares data with user context, you have two problems. |
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Firstly, the current user context can be interrupted by a softirq, and |
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secondly, the critical region could be entered from another CPU. This is |
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where spin_lock_bh() (``include/linux/spinlock.h``) is |
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used. It disables softirqs on that CPU, then grabs the lock. |
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spin_unlock_bh() does the reverse. (The '_bh' suffix is |
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a historical reference to "Bottom Halves", the old name for software |
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interrupts. It should really be called spin_lock_softirq()' in a |
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perfect world). |
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Note that you can also use spin_lock_irq() or |
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spin_lock_irqsave() here, which stop hardware interrupts |
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as well: see `Hard IRQ Context`_. |
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This works perfectly for UP as well: the spin lock vanishes, and this |
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macro simply becomes local_bh_disable() |
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(``include/linux/interrupt.h``), which protects you from the softirq |
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being run. |
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Locking Between User Context and Tasklets |
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----------------------------------------- |
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This is exactly the same as above, because tasklets are actually run |
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from a softirq. |
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Locking Between User Context and Timers |
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--------------------------------------- |
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This, too, is exactly the same as above, because timers are actually run |
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from a softirq. From a locking point of view, tasklets and timers are |
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identical. |
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Locking Between Tasklets/Timers |
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------------------------------- |
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Sometimes a tasklet or timer might want to share data with another |
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tasklet or timer. |
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The Same Tasklet/Timer |
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~~~~~~~~~~~~~~~~~~~~~~ |
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Since a tasklet is never run on two CPUs at once, you don't need to |
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worry about your tasklet being reentrant (running twice at once), even |
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on SMP. |
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Different Tasklets/Timers |
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~~~~~~~~~~~~~~~~~~~~~~~~~ |
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If another tasklet/timer wants to share data with your tasklet or timer |
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, you will both need to use spin_lock() and |
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spin_unlock() calls. spin_lock_bh() is |
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unnecessary here, as you are already in a tasklet, and none will be run |
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on the same CPU. |
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Locking Between Softirqs |
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------------------------ |
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Often a softirq might want to share data with itself or a tasklet/timer. |
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The Same Softirq |
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~~~~~~~~~~~~~~~~ |
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The same softirq can run on the other CPUs: you can use a per-CPU array |
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(see `Per-CPU Data`_) for better performance. If you're |
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going so far as to use a softirq, you probably care about scalable |
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performance enough to justify the extra complexity. |
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You'll need to use spin_lock() and |
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spin_unlock() for shared data. |
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Different Softirqs |
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~~~~~~~~~~~~~~~~~~ |
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You'll need to use spin_lock() and |
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spin_unlock() for shared data, whether it be a timer, |
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tasklet, different softirq or the same or another softirq: any of them |
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could be running on a different CPU. |
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Hard IRQ Context |
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================ |
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Hardware interrupts usually communicate with a tasklet or softirq. |
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Frequently this involves putting work in a queue, which the softirq will |
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take out. |
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Locking Between Hard IRQ and Softirqs/Tasklets |
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---------------------------------------------- |
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If a hardware irq handler shares data with a softirq, you have two |
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concerns. Firstly, the softirq processing can be interrupted by a |
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hardware interrupt, and secondly, the critical region could be entered |
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by a hardware interrupt on another CPU. This is where |
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spin_lock_irq() is used. It is defined to disable |
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interrupts on that cpu, then grab the lock. |
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spin_unlock_irq() does the reverse. |
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The irq handler does not need to use spin_lock_irq(), because |
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the softirq cannot run while the irq handler is running: it can use |
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spin_lock(), which is slightly faster. The only exception |
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would be if a different hardware irq handler uses the same lock: |
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spin_lock_irq() will stop that from interrupting us. |
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This works perfectly for UP as well: the spin lock vanishes, and this |
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macro simply becomes local_irq_disable() |
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(``include/asm/smp.h``), which protects you from the softirq/tasklet/BH |
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being run. |
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spin_lock_irqsave() (``include/linux/spinlock.h``) is a |
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variant which saves whether interrupts were on or off in a flags word, |
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which is passed to spin_unlock_irqrestore(). This means |
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that the same code can be used inside an hard irq handler (where |
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interrupts are already off) and in softirqs (where the irq disabling is |
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required). |
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Note that softirqs (and hence tasklets and timers) are run on return |
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from hardware interrupts, so spin_lock_irq() also stops |
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these. In that sense, spin_lock_irqsave() is the most |
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general and powerful locking function. |
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Locking Between Two Hard IRQ Handlers |
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------------------------------------- |
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It is rare to have to share data between two IRQ handlers, but if you |
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do, spin_lock_irqsave() should be used: it is |
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architecture-specific whether all interrupts are disabled inside irq |
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handlers themselves. |
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Cheat Sheet For Locking |
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======================= |
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Pete Zaitcev gives the following summary: |
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- If you are in a process context (any syscall) and want to lock other |
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process out, use a mutex. You can take a mutex and sleep |
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(``copy_from_user*(`` or ``kmalloc(x,GFP_KERNEL)``). |
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- Otherwise (== data can be touched in an interrupt), use |
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spin_lock_irqsave() and |
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spin_unlock_irqrestore(). |
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- Avoid holding spinlock for more than 5 lines of code and across any |
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function call (except accessors like readb()). |
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Table of Minimum Requirements |
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----------------------------- |
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The following table lists the **minimum** locking requirements between |
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various contexts. In some cases, the same context can only be running on |
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one CPU at a time, so no locking is required for that context (eg. a |
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particular thread can only run on one CPU at a time, but if it needs |
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shares data with another thread, locking is required). |
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Remember the advice above: you can always use |
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spin_lock_irqsave(), which is a superset of all other |
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spinlock primitives. |
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============== ============= ============= ========= ========= ========= ========= ======= ======= ============== ============== |
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. IRQ Handler A IRQ Handler B Softirq A Softirq B Tasklet A Tasklet B Timer A Timer B User Context A User Context B |
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============== ============= ============= ========= ========= ========= ========= ======= ======= ============== ============== |
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IRQ Handler A None |
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IRQ Handler B SLIS None |
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Softirq A SLI SLI SL |
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Softirq B SLI SLI SL SL |
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Tasklet A SLI SLI SL SL None |
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Tasklet B SLI SLI SL SL SL None |
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Timer A SLI SLI SL SL SL SL None |
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Timer B SLI SLI SL SL SL SL SL None |
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User Context A SLI SLI SLBH SLBH SLBH SLBH SLBH SLBH None |
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User Context B SLI SLI SLBH SLBH SLBH SLBH SLBH SLBH MLI None |
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============== ============= ============= ========= ========= ========= ========= ======= ======= ============== ============== |
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Table: Table of Locking Requirements |
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+--------+----------------------------+ |
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| SLIS | spin_lock_irqsave | |
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+--------+----------------------------+ |
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| SLI | spin_lock_irq | |
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+--------+----------------------------+ |
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| SL | spin_lock | |
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+--------+----------------------------+ |
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| SLBH | spin_lock_bh | |
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+--------+----------------------------+ |
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| MLI | mutex_lock_interruptible | |
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+--------+----------------------------+ |
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Table: Legend for Locking Requirements Table |
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The trylock Functions |
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===================== |
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There are functions that try to acquire a lock only once and immediately |
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return a value telling about success or failure to acquire the lock. |
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They can be used if you need no access to the data protected with the |
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lock when some other thread is holding the lock. You should acquire the |
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lock later if you then need access to the data protected with the lock. |
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spin_trylock() does not spin but returns non-zero if it |
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acquires the spinlock on the first try or 0 if not. This function can be |
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used in all contexts like spin_lock(): you must have |
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disabled the contexts that might interrupt you and acquire the spin |
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lock. |
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mutex_trylock() does not suspend your task but returns |
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non-zero if it could lock the mutex on the first try or 0 if not. This |
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function cannot be safely used in hardware or software interrupt |
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contexts despite not sleeping. |
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Common Examples |
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=============== |
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Let's step through a simple example: a cache of number to name mappings. |
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The cache keeps a count of how often each of the objects is used, and |
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when it gets full, throws out the least used one. |
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All In User Context |
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------------------- |
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For our first example, we assume that all operations are in user context |
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(ie. from system calls), so we can sleep. This means we can use a mutex |
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to protect the cache and all the objects within it. Here's the code:: |
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#include <linux/list.h> |
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#include <linux/slab.h> |
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#include <linux/string.h> |
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#include <linux/mutex.h> |
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#include <asm/errno.h> |
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struct object |
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{ |
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struct list_head list; |
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int id; |
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char name[32]; |
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int popularity; |
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}; |
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/* Protects the cache, cache_num, and the objects within it */ |
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static DEFINE_MUTEX(cache_lock); |
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static LIST_HEAD(cache); |
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static unsigned int cache_num = 0; |
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#define MAX_CACHE_SIZE 10 |
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|
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/* Must be holding cache_lock */ |
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static struct object *__cache_find(int id) |
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{ |
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struct object *i; |
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|
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list_for_each_entry(i, &cache, list) |
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if (i->id == id) { |
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i->popularity++; |
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return i; |
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} |
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return NULL; |
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} |
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|
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/* Must be holding cache_lock */ |
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static void __cache_delete(struct object *obj) |
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{ |
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BUG_ON(!obj); |
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list_del(&obj->list); |
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kfree(obj); |
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cache_num--; |
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} |
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|
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/* Must be holding cache_lock */ |
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static void __cache_add(struct object *obj) |
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{ |
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list_add(&obj->list, &cache); |
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if (++cache_num > MAX_CACHE_SIZE) { |
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struct object *i, *outcast = NULL; |
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list_for_each_entry(i, &cache, list) { |
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if (!outcast || i->popularity < outcast->popularity) |
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outcast = i; |
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} |
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__cache_delete(outcast); |
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} |
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} |
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int cache_add(int id, const char *name) |
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{ |
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struct object *obj; |
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|
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if ((obj = kmalloc(sizeof(*obj), GFP_KERNEL)) == NULL) |
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return -ENOMEM; |
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strscpy(obj->name, name, sizeof(obj->name)); |
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obj->id = id; |
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obj->popularity = 0; |
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mutex_lock(&cache_lock); |
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__cache_add(obj); |
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mutex_unlock(&cache_lock); |
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return 0; |
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} |
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void cache_delete(int id) |
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{ |
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mutex_lock(&cache_lock); |
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__cache_delete(__cache_find(id)); |
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mutex_unlock(&cache_lock); |
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} |
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int cache_find(int id, char *name) |
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{ |
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struct object *obj; |
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int ret = -ENOENT; |
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|
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mutex_lock(&cache_lock); |
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obj = __cache_find(id); |
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if (obj) { |
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ret = 0; |
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strcpy(name, obj->name); |
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} |
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mutex_unlock(&cache_lock); |
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return ret; |
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} |
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|
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Note that we always make sure we have the cache_lock when we add, |
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delete, or look up the cache: both the cache infrastructure itself and |
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the contents of the objects are protected by the lock. In this case it's |
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easy, since we copy the data for the user, and never let them access the |
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objects directly. |
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|
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There is a slight (and common) optimization here: in |
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cache_add() we set up the fields of the object before |
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grabbing the lock. This is safe, as no-one else can access it until we |
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put it in cache. |
|
|
|
Accessing From Interrupt Context |
|
-------------------------------- |
|
|
|
Now consider the case where cache_find() can be called |
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from interrupt context: either a hardware interrupt or a softirq. An |
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example would be a timer which deletes object from the cache. |
|
|
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The change is shown below, in standard patch format: the ``-`` are lines |
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which are taken away, and the ``+`` are lines which are added. |
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|
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:: |
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|
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--- cache.c.usercontext 2003-12-09 13:58:54.000000000 +1100 |
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+++ cache.c.interrupt 2003-12-09 14:07:49.000000000 +1100 |
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@@ -12,7 +12,7 @@ |
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int popularity; |
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}; |
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|
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-static DEFINE_MUTEX(cache_lock); |
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+static DEFINE_SPINLOCK(cache_lock); |
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static LIST_HEAD(cache); |
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static unsigned int cache_num = 0; |
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#define MAX_CACHE_SIZE 10 |
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@@ -55,6 +55,7 @@ |
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int cache_add(int id, const char *name) |
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{ |
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struct object *obj; |
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+ unsigned long flags; |
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|
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if ((obj = kmalloc(sizeof(*obj), GFP_KERNEL)) == NULL) |
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return -ENOMEM; |
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@@ -63,30 +64,33 @@ |
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obj->id = id; |
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obj->popularity = 0; |
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|
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- mutex_lock(&cache_lock); |
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+ spin_lock_irqsave(&cache_lock, flags); |
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__cache_add(obj); |
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- mutex_unlock(&cache_lock); |
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+ spin_unlock_irqrestore(&cache_lock, flags); |
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return 0; |
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} |
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|
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void cache_delete(int id) |
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{ |
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- mutex_lock(&cache_lock); |
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+ unsigned long flags; |
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+ |
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+ spin_lock_irqsave(&cache_lock, flags); |
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__cache_delete(__cache_find(id)); |
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- mutex_unlock(&cache_lock); |
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+ spin_unlock_irqrestore(&cache_lock, flags); |
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} |
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|
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int cache_find(int id, char *name) |
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{ |
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struct object *obj; |
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int ret = -ENOENT; |
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+ unsigned long flags; |
|
|
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- mutex_lock(&cache_lock); |
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+ spin_lock_irqsave(&cache_lock, flags); |
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obj = __cache_find(id); |
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if (obj) { |
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ret = 0; |
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strcpy(name, obj->name); |
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} |
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- mutex_unlock(&cache_lock); |
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+ spin_unlock_irqrestore(&cache_lock, flags); |
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return ret; |
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} |
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|
|
Note that the spin_lock_irqsave() will turn off |
|
interrupts if they are on, otherwise does nothing (if we are already in |
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an interrupt handler), hence these functions are safe to call from any |
|
context. |
|
|
|
Unfortunately, cache_add() calls kmalloc() |
|
with the ``GFP_KERNEL`` flag, which is only legal in user context. I |
|
have assumed that cache_add() is still only called in |
|
user context, otherwise this should become a parameter to |
|
cache_add(). |
|
|
|
Exposing Objects Outside This File |
|
---------------------------------- |
|
|
|
If our objects contained more information, it might not be sufficient to |
|
copy the information in and out: other parts of the code might want to |
|
keep pointers to these objects, for example, rather than looking up the |
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id every time. This produces two problems. |
|
|
|
The first problem is that we use the ``cache_lock`` to protect objects: |
|
we'd need to make this non-static so the rest of the code can use it. |
|
This makes locking trickier, as it is no longer all in one place. |
|
|
|
The second problem is the lifetime problem: if another structure keeps a |
|
pointer to an object, it presumably expects that pointer to remain |
|
valid. Unfortunately, this is only guaranteed while you hold the lock, |
|
otherwise someone might call cache_delete() and even |
|
worse, add another object, re-using the same address. |
|
|
|
As there is only one lock, you can't hold it forever: no-one else would |
|
get any work done. |
|
|
|
The solution to this problem is to use a reference count: everyone who |
|
has a pointer to the object increases it when they first get the object, |
|
and drops the reference count when they're finished with it. Whoever |
|
drops it to zero knows it is unused, and can actually delete it. |
|
|
|
Here is the code:: |
|
|
|
--- cache.c.interrupt 2003-12-09 14:25:43.000000000 +1100 |
|
+++ cache.c.refcnt 2003-12-09 14:33:05.000000000 +1100 |
|
@@ -7,6 +7,7 @@ |
|
struct object |
|
{ |
|
struct list_head list; |
|
+ unsigned int refcnt; |
|
int id; |
|
char name[32]; |
|
int popularity; |
|
@@ -17,6 +18,35 @@ |
|
static unsigned int cache_num = 0; |
|
#define MAX_CACHE_SIZE 10 |
|
|
|
+static void __object_put(struct object *obj) |
|
+{ |
|
+ if (--obj->refcnt == 0) |
|
+ kfree(obj); |
|
+} |
|
+ |
|
+static void __object_get(struct object *obj) |
|
+{ |
|
+ obj->refcnt++; |
|
+} |
|
+ |
|
+void object_put(struct object *obj) |
|
+{ |
|
+ unsigned long flags; |
|
+ |
|
+ spin_lock_irqsave(&cache_lock, flags); |
|
+ __object_put(obj); |
|
+ spin_unlock_irqrestore(&cache_lock, flags); |
|
+} |
|
+ |
|
+void object_get(struct object *obj) |
|
+{ |
|
+ unsigned long flags; |
|
+ |
|
+ spin_lock_irqsave(&cache_lock, flags); |
|
+ __object_get(obj); |
|
+ spin_unlock_irqrestore(&cache_lock, flags); |
|
+} |
|
+ |
|
/* Must be holding cache_lock */ |
|
static struct object *__cache_find(int id) |
|
{ |
|
@@ -35,6 +65,7 @@ |
|
{ |
|
BUG_ON(!obj); |
|
list_del(&obj->list); |
|
+ __object_put(obj); |
|
cache_num--; |
|
} |
|
|
|
@@ -63,6 +94,7 @@ |
|
strscpy(obj->name, name, sizeof(obj->name)); |
|
obj->id = id; |
|
obj->popularity = 0; |
|
+ obj->refcnt = 1; /* The cache holds a reference */ |
|
|
|
spin_lock_irqsave(&cache_lock, flags); |
|
__cache_add(obj); |
|
@@ -79,18 +111,15 @@ |
|
spin_unlock_irqrestore(&cache_lock, flags); |
|
} |
|
|
|
-int cache_find(int id, char *name) |
|
+struct object *cache_find(int id) |
|
{ |
|
struct object *obj; |
|
- int ret = -ENOENT; |
|
unsigned long flags; |
|
|
|
spin_lock_irqsave(&cache_lock, flags); |
|
obj = __cache_find(id); |
|
- if (obj) { |
|
- ret = 0; |
|
- strcpy(name, obj->name); |
|
- } |
|
+ if (obj) |
|
+ __object_get(obj); |
|
spin_unlock_irqrestore(&cache_lock, flags); |
|
- return ret; |
|
+ return obj; |
|
} |
|
|
|
We encapsulate the reference counting in the standard 'get' and 'put' |
|
functions. Now we can return the object itself from |
|
cache_find() which has the advantage that the user can |
|
now sleep holding the object (eg. to copy_to_user() to |
|
name to userspace). |
|
|
|
The other point to note is that I said a reference should be held for |
|
every pointer to the object: thus the reference count is 1 when first |
|
inserted into the cache. In some versions the framework does not hold a |
|
reference count, but they are more complicated. |
|
|
|
Using Atomic Operations For The Reference Count |
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
|
|
|
In practice, :c:type:`atomic_t` would usually be used for refcnt. There are a |
|
number of atomic operations defined in ``include/asm/atomic.h``: these |
|
are guaranteed to be seen atomically from all CPUs in the system, so no |
|
lock is required. In this case, it is simpler than using spinlocks, |
|
although for anything non-trivial using spinlocks is clearer. The |
|
atomic_inc() and atomic_dec_and_test() |
|
are used instead of the standard increment and decrement operators, and |
|
the lock is no longer used to protect the reference count itself. |
|
|
|
:: |
|
|
|
--- cache.c.refcnt 2003-12-09 15:00:35.000000000 +1100 |
|
+++ cache.c.refcnt-atomic 2003-12-11 15:49:42.000000000 +1100 |
|
@@ -7,7 +7,7 @@ |
|
struct object |
|
{ |
|
struct list_head list; |
|
- unsigned int refcnt; |
|
+ atomic_t refcnt; |
|
int id; |
|
char name[32]; |
|
int popularity; |
|
@@ -18,33 +18,15 @@ |
|
static unsigned int cache_num = 0; |
|
#define MAX_CACHE_SIZE 10 |
|
|
|
-static void __object_put(struct object *obj) |
|
-{ |
|
- if (--obj->refcnt == 0) |
|
- kfree(obj); |
|
-} |
|
- |
|
-static void __object_get(struct object *obj) |
|
-{ |
|
- obj->refcnt++; |
|
-} |
|
- |
|
void object_put(struct object *obj) |
|
{ |
|
- unsigned long flags; |
|
- |
|
- spin_lock_irqsave(&cache_lock, flags); |
|
- __object_put(obj); |
|
- spin_unlock_irqrestore(&cache_lock, flags); |
|
+ if (atomic_dec_and_test(&obj->refcnt)) |
|
+ kfree(obj); |
|
} |
|
|
|
void object_get(struct object *obj) |
|
{ |
|
- unsigned long flags; |
|
- |
|
- spin_lock_irqsave(&cache_lock, flags); |
|
- __object_get(obj); |
|
- spin_unlock_irqrestore(&cache_lock, flags); |
|
+ atomic_inc(&obj->refcnt); |
|
} |
|
|
|
/* Must be holding cache_lock */ |
|
@@ -65,7 +47,7 @@ |
|
{ |
|
BUG_ON(!obj); |
|
list_del(&obj->list); |
|
- __object_put(obj); |
|
+ object_put(obj); |
|
cache_num--; |
|
} |
|
|
|
@@ -94,7 +76,7 @@ |
|
strscpy(obj->name, name, sizeof(obj->name)); |
|
obj->id = id; |
|
obj->popularity = 0; |
|
- obj->refcnt = 1; /* The cache holds a reference */ |
|
+ atomic_set(&obj->refcnt, 1); /* The cache holds a reference */ |
|
|
|
spin_lock_irqsave(&cache_lock, flags); |
|
__cache_add(obj); |
|
@@ -119,7 +101,7 @@ |
|
spin_lock_irqsave(&cache_lock, flags); |
|
obj = __cache_find(id); |
|
if (obj) |
|
- __object_get(obj); |
|
+ object_get(obj); |
|
spin_unlock_irqrestore(&cache_lock, flags); |
|
return obj; |
|
} |
|
|
|
Protecting The Objects Themselves |
|
--------------------------------- |
|
|
|
In these examples, we assumed that the objects (except the reference |
|
counts) never changed once they are created. If we wanted to allow the |
|
name to change, there are three possibilities: |
|
|
|
- You can make ``cache_lock`` non-static, and tell people to grab that |
|
lock before changing the name in any object. |
|
|
|
- You can provide a cache_obj_rename() which grabs this |
|
lock and changes the name for the caller, and tell everyone to use |
|
that function. |
|
|
|
- You can make the ``cache_lock`` protect only the cache itself, and |
|
use another lock to protect the name. |
|
|
|
Theoretically, you can make the locks as fine-grained as one lock for |
|
every field, for every object. In practice, the most common variants |
|
are: |
|
|
|
- One lock which protects the infrastructure (the ``cache`` list in |
|
this example) and all the objects. This is what we have done so far. |
|
|
|
- One lock which protects the infrastructure (including the list |
|
pointers inside the objects), and one lock inside the object which |
|
protects the rest of that object. |
|
|
|
- Multiple locks to protect the infrastructure (eg. one lock per hash |
|
chain), possibly with a separate per-object lock. |
|
|
|
Here is the "lock-per-object" implementation: |
|
|
|
:: |
|
|
|
--- cache.c.refcnt-atomic 2003-12-11 15:50:54.000000000 +1100 |
|
+++ cache.c.perobjectlock 2003-12-11 17:15:03.000000000 +1100 |
|
@@ -6,11 +6,17 @@ |
|
|
|
struct object |
|
{ |
|
+ /* These two protected by cache_lock. */ |
|
struct list_head list; |
|
+ int popularity; |
|
+ |
|
atomic_t refcnt; |
|
+ |
|
+ /* Doesn't change once created. */ |
|
int id; |
|
+ |
|
+ spinlock_t lock; /* Protects the name */ |
|
char name[32]; |
|
- int popularity; |
|
}; |
|
|
|
static DEFINE_SPINLOCK(cache_lock); |
|
@@ -77,6 +84,7 @@ |
|
obj->id = id; |
|
obj->popularity = 0; |
|
atomic_set(&obj->refcnt, 1); /* The cache holds a reference */ |
|
+ spin_lock_init(&obj->lock); |
|
|
|
spin_lock_irqsave(&cache_lock, flags); |
|
__cache_add(obj); |
|
|
|
Note that I decide that the popularity count should be protected by the |
|
``cache_lock`` rather than the per-object lock: this is because it (like |
|
the :c:type:`struct list_head <list_head>` inside the object) |
|
is logically part of the infrastructure. This way, I don't need to grab |
|
the lock of every object in __cache_add() when seeking |
|
the least popular. |
|
|
|
I also decided that the id member is unchangeable, so I don't need to |
|
grab each object lock in __cache_find() to examine the |
|
id: the object lock is only used by a caller who wants to read or write |
|
the name field. |
|
|
|
Note also that I added a comment describing what data was protected by |
|
which locks. This is extremely important, as it describes the runtime |
|
behavior of the code, and can be hard to gain from just reading. And as |
|
Alan Cox says, “Lock data, not code”. |
|
|
|
Common Problems |
|
=============== |
|
|
|
Deadlock: Simple and Advanced |
|
----------------------------- |
|
|
|
There is a coding bug where a piece of code tries to grab a spinlock |
|
twice: it will spin forever, waiting for the lock to be released |
|
(spinlocks, rwlocks and mutexes are not recursive in Linux). This is |
|
trivial to diagnose: not a |
|
stay-up-five-nights-talk-to-fluffy-code-bunnies kind of problem. |
|
|
|
For a slightly more complex case, imagine you have a region shared by a |
|
softirq and user context. If you use a spin_lock() call |
|
to protect it, it is possible that the user context will be interrupted |
|
by the softirq while it holds the lock, and the softirq will then spin |
|
forever trying to get the same lock. |
|
|
|
Both of these are called deadlock, and as shown above, it can occur even |
|
with a single CPU (although not on UP compiles, since spinlocks vanish |
|
on kernel compiles with ``CONFIG_SMP``\ =n. You'll still get data |
|
corruption in the second example). |
|
|
|
This complete lockup is easy to diagnose: on SMP boxes the watchdog |
|
timer or compiling with ``DEBUG_SPINLOCK`` set |
|
(``include/linux/spinlock.h``) will show this up immediately when it |
|
happens. |
|
|
|
A more complex problem is the so-called 'deadly embrace', involving two |
|
or more locks. Say you have a hash table: each entry in the table is a |
|
spinlock, and a chain of hashed objects. Inside a softirq handler, you |
|
sometimes want to alter an object from one place in the hash to another: |
|
you grab the spinlock of the old hash chain and the spinlock of the new |
|
hash chain, and delete the object from the old one, and insert it in the |
|
new one. |
|
|
|
There are two problems here. First, if your code ever tries to move the |
|
object to the same chain, it will deadlock with itself as it tries to |
|
lock it twice. Secondly, if the same softirq on another CPU is trying to |
|
move another object in the reverse direction, the following could |
|
happen: |
|
|
|
+-----------------------+-----------------------+ |
|
| CPU 1 | CPU 2 | |
|
+=======================+=======================+ |
|
| Grab lock A -> OK | Grab lock B -> OK | |
|
+-----------------------+-----------------------+ |
|
| Grab lock B -> spin | Grab lock A -> spin | |
|
+-----------------------+-----------------------+ |
|
|
|
Table: Consequences |
|
|
|
The two CPUs will spin forever, waiting for the other to give up their |
|
lock. It will look, smell, and feel like a crash. |
|
|
|
Preventing Deadlock |
|
------------------- |
|
|
|
Textbooks will tell you that if you always lock in the same order, you |
|
will never get this kind of deadlock. Practice will tell you that this |
|
approach doesn't scale: when I create a new lock, I don't understand |
|
enough of the kernel to figure out where in the 5000 lock hierarchy it |
|
will fit. |
|
|
|
The best locks are encapsulated: they never get exposed in headers, and |
|
are never held around calls to non-trivial functions outside the same |
|
file. You can read through this code and see that it will never |
|
deadlock, because it never tries to grab another lock while it has that |
|
one. People using your code don't even need to know you are using a |
|
lock. |
|
|
|
A classic problem here is when you provide callbacks or hooks: if you |
|
call these with the lock held, you risk simple deadlock, or a deadly |
|
embrace (who knows what the callback will do?). Remember, the other |
|
programmers are out to get you, so don't do this. |
|
|
|
Overzealous Prevention Of Deadlocks |
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
|
|
|
Deadlocks are problematic, but not as bad as data corruption. Code which |
|
grabs a read lock, searches a list, fails to find what it wants, drops |
|
the read lock, grabs a write lock and inserts the object has a race |
|
condition. |
|
|
|
If you don't see why, please stay away from my code. |
|
|
|
Racing Timers: A Kernel Pastime |
|
------------------------------- |
|
|
|
Timers can produce their own special problems with races. Consider a |
|
collection of objects (list, hash, etc) where each object has a timer |
|
which is due to destroy it. |
|
|
|
If you want to destroy the entire collection (say on module removal), |
|
you might do the following:: |
|
|
|
/* THIS CODE BAD BAD BAD BAD: IF IT WAS ANY WORSE IT WOULD USE |
|
HUNGARIAN NOTATION */ |
|
spin_lock_bh(&list_lock); |
|
|
|
while (list) { |
|
struct foo *next = list->next; |
|
del_timer(&list->timer); |
|
kfree(list); |
|
list = next; |
|
} |
|
|
|
spin_unlock_bh(&list_lock); |
|
|
|
|
|
Sooner or later, this will crash on SMP, because a timer can have just |
|
gone off before the spin_lock_bh(), and it will only get |
|
the lock after we spin_unlock_bh(), and then try to free |
|
the element (which has already been freed!). |
|
|
|
This can be avoided by checking the result of |
|
del_timer(): if it returns 1, the timer has been deleted. |
|
If 0, it means (in this case) that it is currently running, so we can |
|
do:: |
|
|
|
retry: |
|
spin_lock_bh(&list_lock); |
|
|
|
while (list) { |
|
struct foo *next = list->next; |
|
if (!del_timer(&list->timer)) { |
|
/* Give timer a chance to delete this */ |
|
spin_unlock_bh(&list_lock); |
|
goto retry; |
|
} |
|
kfree(list); |
|
list = next; |
|
} |
|
|
|
spin_unlock_bh(&list_lock); |
|
|
|
|
|
Another common problem is deleting timers which restart themselves (by |
|
calling add_timer() at the end of their timer function). |
|
Because this is a fairly common case which is prone to races, you should |
|
use del_timer_sync() (``include/linux/timer.h``) to |
|
handle this case. It returns the number of times the timer had to be |
|
deleted before we finally stopped it from adding itself back in. |
|
|
|
Locking Speed |
|
============= |
|
|
|
There are three main things to worry about when considering speed of |
|
some code which does locking. First is concurrency: how many things are |
|
going to be waiting while someone else is holding a lock. Second is the |
|
time taken to actually acquire and release an uncontended lock. Third is |
|
using fewer, or smarter locks. I'm assuming that the lock is used fairly |
|
often: otherwise, you wouldn't be concerned about efficiency. |
|
|
|
Concurrency depends on how long the lock is usually held: you should |
|
hold the lock for as long as needed, but no longer. In the cache |
|
example, we always create the object without the lock held, and then |
|
grab the lock only when we are ready to insert it in the list. |
|
|
|
Acquisition times depend on how much damage the lock operations do to |
|
the pipeline (pipeline stalls) and how likely it is that this CPU was |
|
the last one to grab the lock (ie. is the lock cache-hot for this CPU): |
|
on a machine with more CPUs, this likelihood drops fast. Consider a |
|
700MHz Intel Pentium III: an instruction takes about 0.7ns, an atomic |
|
increment takes about 58ns, a lock which is cache-hot on this CPU takes |
|
160ns, and a cacheline transfer from another CPU takes an additional 170 |
|
to 360ns. (These figures from Paul McKenney's `Linux Journal RCU |
|
article <http://www.linuxjournal.com/article.php?sid=6993>`__). |
|
|
|
These two aims conflict: holding a lock for a short time might be done |
|
by splitting locks into parts (such as in our final per-object-lock |
|
example), but this increases the number of lock acquisitions, and the |
|
results are often slower than having a single lock. This is another |
|
reason to advocate locking simplicity. |
|
|
|
The third concern is addressed below: there are some methods to reduce |
|
the amount of locking which needs to be done. |
|
|
|
Read/Write Lock Variants |
|
------------------------ |
|
|
|
Both spinlocks and mutexes have read/write variants: ``rwlock_t`` and |
|
:c:type:`struct rw_semaphore <rw_semaphore>`. These divide |
|
users into two classes: the readers and the writers. If you are only |
|
reading the data, you can get a read lock, but to write to the data you |
|
need the write lock. Many people can hold a read lock, but a writer must |
|
be sole holder. |
|
|
|
If your code divides neatly along reader/writer lines (as our cache code |
|
does), and the lock is held by readers for significant lengths of time, |
|
using these locks can help. They are slightly slower than the normal |
|
locks though, so in practice ``rwlock_t`` is not usually worthwhile. |
|
|
|
Avoiding Locks: Read Copy Update |
|
-------------------------------- |
|
|
|
There is a special method of read/write locking called Read Copy Update. |
|
Using RCU, the readers can avoid taking a lock altogether: as we expect |
|
our cache to be read more often than updated (otherwise the cache is a |
|
waste of time), it is a candidate for this optimization. |
|
|
|
How do we get rid of read locks? Getting rid of read locks means that |
|
writers may be changing the list underneath the readers. That is |
|
actually quite simple: we can read a linked list while an element is |
|
being added if the writer adds the element very carefully. For example, |
|
adding ``new`` to a single linked list called ``list``:: |
|
|
|
new->next = list->next; |
|
wmb(); |
|
list->next = new; |
|
|
|
|
|
The wmb() is a write memory barrier. It ensures that the |
|
first operation (setting the new element's ``next`` pointer) is complete |
|
and will be seen by all CPUs, before the second operation is (putting |
|
the new element into the list). This is important, since modern |
|
compilers and modern CPUs can both reorder instructions unless told |
|
otherwise: we want a reader to either not see the new element at all, or |
|
see the new element with the ``next`` pointer correctly pointing at the |
|
rest of the list. |
|
|
|
Fortunately, there is a function to do this for standard |
|
:c:type:`struct list_head <list_head>` lists: |
|
list_add_rcu() (``include/linux/list.h``). |
|
|
|
Removing an element from the list is even simpler: we replace the |
|
pointer to the old element with a pointer to its successor, and readers |
|
will either see it, or skip over it. |
|
|
|
:: |
|
|
|
list->next = old->next; |
|
|
|
|
|
There is list_del_rcu() (``include/linux/list.h``) which |
|
does this (the normal version poisons the old object, which we don't |
|
want). |
|
|
|
The reader must also be careful: some CPUs can look through the ``next`` |
|
pointer to start reading the contents of the next element early, but |
|
don't realize that the pre-fetched contents is wrong when the ``next`` |
|
pointer changes underneath them. Once again, there is a |
|
list_for_each_entry_rcu() (``include/linux/list.h``) |
|
to help you. Of course, writers can just use |
|
list_for_each_entry(), since there cannot be two |
|
simultaneous writers. |
|
|
|
Our final dilemma is this: when can we actually destroy the removed |
|
element? Remember, a reader might be stepping through this element in |
|
the list right now: if we free this element and the ``next`` pointer |
|
changes, the reader will jump off into garbage and crash. We need to |
|
wait until we know that all the readers who were traversing the list |
|
when we deleted the element are finished. We use |
|
call_rcu() to register a callback which will actually |
|
destroy the object once all pre-existing readers are finished. |
|
Alternatively, synchronize_rcu() may be used to block |
|
until all pre-existing are finished. |
|
|
|
But how does Read Copy Update know when the readers are finished? The |
|
method is this: firstly, the readers always traverse the list inside |
|
rcu_read_lock()/rcu_read_unlock() pairs: |
|
these simply disable preemption so the reader won't go to sleep while |
|
reading the list. |
|
|
|
RCU then waits until every other CPU has slept at least once: since |
|
readers cannot sleep, we know that any readers which were traversing the |
|
list during the deletion are finished, and the callback is triggered. |
|
The real Read Copy Update code is a little more optimized than this, but |
|
this is the fundamental idea. |
|
|
|
:: |
|
|
|
--- cache.c.perobjectlock 2003-12-11 17:15:03.000000000 +1100 |
|
+++ cache.c.rcupdate 2003-12-11 17:55:14.000000000 +1100 |
|
@@ -1,15 +1,18 @@ |
|
#include <linux/list.h> |
|
#include <linux/slab.h> |
|
#include <linux/string.h> |
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+#include <linux/rcupdate.h> |
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#include <linux/mutex.h> |
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#include <asm/errno.h> |
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|
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struct object |
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{ |
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- /* These two protected by cache_lock. */ |
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+ /* This is protected by RCU */ |
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struct list_head list; |
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int popularity; |
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+ struct rcu_head rcu; |
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+ |
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atomic_t refcnt; |
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|
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/* Doesn't change once created. */ |
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@@ -40,7 +43,7 @@ |
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{ |
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struct object *i; |
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- list_for_each_entry(i, &cache, list) { |
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+ list_for_each_entry_rcu(i, &cache, list) { |
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if (i->id == id) { |
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i->popularity++; |
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return i; |
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@@ -49,19 +52,25 @@ |
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return NULL; |
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} |
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|
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+/* Final discard done once we know no readers are looking. */ |
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+static void cache_delete_rcu(void *arg) |
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+{ |
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+ object_put(arg); |
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+} |
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+ |
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/* Must be holding cache_lock */ |
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static void __cache_delete(struct object *obj) |
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{ |
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BUG_ON(!obj); |
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- list_del(&obj->list); |
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- object_put(obj); |
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+ list_del_rcu(&obj->list); |
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cache_num--; |
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+ call_rcu(&obj->rcu, cache_delete_rcu); |
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} |
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|
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/* Must be holding cache_lock */ |
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static void __cache_add(struct object *obj) |
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{ |
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- list_add(&obj->list, &cache); |
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+ list_add_rcu(&obj->list, &cache); |
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if (++cache_num > MAX_CACHE_SIZE) { |
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struct object *i, *outcast = NULL; |
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list_for_each_entry(i, &cache, list) { |
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@@ -104,12 +114,11 @@ |
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struct object *cache_find(int id) |
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{ |
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struct object *obj; |
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- unsigned long flags; |
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- spin_lock_irqsave(&cache_lock, flags); |
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+ rcu_read_lock(); |
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obj = __cache_find(id); |
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if (obj) |
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object_get(obj); |
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- spin_unlock_irqrestore(&cache_lock, flags); |
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+ rcu_read_unlock(); |
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return obj; |
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} |
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|
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Note that the reader will alter the popularity member in |
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__cache_find(), and now it doesn't hold a lock. One |
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solution would be to make it an ``atomic_t``, but for this usage, we |
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don't really care about races: an approximate result is good enough, so |
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I didn't change it. |
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|
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The result is that cache_find() requires no |
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synchronization with any other functions, so is almost as fast on SMP as |
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it would be on UP. |
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|
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There is a further optimization possible here: remember our original |
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cache code, where there were no reference counts and the caller simply |
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held the lock whenever using the object? This is still possible: if you |
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hold the lock, no one can delete the object, so you don't need to get |
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and put the reference count. |
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|
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Now, because the 'read lock' in RCU is simply disabling preemption, a |
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caller which always has preemption disabled between calling |
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cache_find() and object_put() does not |
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need to actually get and put the reference count: we could expose |
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__cache_find() by making it non-static, and such |
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callers could simply call that. |
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|
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The benefit here is that the reference count is not written to: the |
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object is not altered in any way, which is much faster on SMP machines |
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due to caching. |
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Per-CPU Data |
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------------ |
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|
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Another technique for avoiding locking which is used fairly widely is to |
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duplicate information for each CPU. For example, if you wanted to keep a |
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count of a common condition, you could use a spin lock and a single |
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counter. Nice and simple. |
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|
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If that was too slow (it's usually not, but if you've got a really big |
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machine to test on and can show that it is), you could instead use a |
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counter for each CPU, then none of them need an exclusive lock. See |
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DEFINE_PER_CPU(), get_cpu_var() and |
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put_cpu_var() (``include/linux/percpu.h``). |
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|
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Of particular use for simple per-cpu counters is the ``local_t`` type, |
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and the cpu_local_inc() and related functions, which are |
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more efficient than simple code on some architectures |
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(``include/asm/local.h``). |
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|
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Note that there is no simple, reliable way of getting an exact value of |
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such a counter, without introducing more locks. This is not a problem |
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for some uses. |
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|
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Data Which Mostly Used By An IRQ Handler |
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---------------------------------------- |
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|
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If data is always accessed from within the same IRQ handler, you don't |
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need a lock at all: the kernel already guarantees that the irq handler |
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will not run simultaneously on multiple CPUs. |
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|
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Manfred Spraul points out that you can still do this, even if the data |
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is very occasionally accessed in user context or softirqs/tasklets. The |
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irq handler doesn't use a lock, and all other accesses are done as so:: |
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|
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spin_lock(&lock); |
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disable_irq(irq); |
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... |
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enable_irq(irq); |
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spin_unlock(&lock); |
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|
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The disable_irq() prevents the irq handler from running |
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(and waits for it to finish if it's currently running on other CPUs). |
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The spinlock prevents any other accesses happening at the same time. |
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Naturally, this is slower than just a spin_lock_irq() |
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call, so it only makes sense if this type of access happens extremely |
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rarely. |
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|
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What Functions Are Safe To Call From Interrupts? |
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================================================ |
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|
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Many functions in the kernel sleep (ie. call schedule()) directly or |
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indirectly: you can never call them while holding a spinlock, or with |
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preemption disabled. This also means you need to be in user context: |
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calling them from an interrupt is illegal. |
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|
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Some Functions Which Sleep |
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-------------------------- |
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The most common ones are listed below, but you usually have to read the |
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code to find out if other calls are safe. If everyone else who calls it |
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can sleep, you probably need to be able to sleep, too. In particular, |
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registration and deregistration functions usually expect to be called |
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from user context, and can sleep. |
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|
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- Accesses to userspace: |
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- copy_from_user() |
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- copy_to_user() |
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- get_user() |
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- put_user() |
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- kmalloc(GP_KERNEL) <kmalloc>` |
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|
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- mutex_lock_interruptible() and |
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mutex_lock() |
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|
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There is a mutex_trylock() which does not sleep. |
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Still, it must not be used inside interrupt context since its |
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implementation is not safe for that. mutex_unlock() |
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will also never sleep. It cannot be used in interrupt context either |
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since a mutex must be released by the same task that acquired it. |
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|
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Some Functions Which Don't Sleep |
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-------------------------------- |
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Some functions are safe to call from any context, or holding almost any |
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lock. |
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- printk() |
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- kfree() |
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- add_timer() and del_timer() |
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Mutex API reference |
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=================== |
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|
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.. kernel-doc:: include/linux/mutex.h |
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:internal: |
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|
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.. kernel-doc:: kernel/locking/mutex.c |
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:export: |
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Futex API reference |
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=================== |
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|
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.. kernel-doc:: kernel/futex.c |
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:internal: |
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Further reading |
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=============== |
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|
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- ``Documentation/locking/spinlocks.rst``: Linus Torvalds' spinlocking |
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tutorial in the kernel sources. |
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|
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- Unix Systems for Modern Architectures: Symmetric Multiprocessing and |
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Caching for Kernel Programmers: |
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|
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Curt Schimmel's very good introduction to kernel level locking (not |
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written for Linux, but nearly everything applies). The book is |
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expensive, but really worth every penny to understand SMP locking. |
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[ISBN: 0201633388] |
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|
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Thanks |
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====== |
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|
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Thanks to Telsa Gwynne for DocBooking, neatening and adding style. |
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|
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Thanks to Martin Pool, Philipp Rumpf, Stephen Rothwell, Paul Mackerras, |
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Ruedi Aschwanden, Alan Cox, Manfred Spraul, Tim Waugh, Pete Zaitcev, |
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James Morris, Robert Love, Paul McKenney, John Ashby for proofreading, |
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correcting, flaming, commenting. |
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|
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Thanks to the cabal for having no influence on this document. |
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|
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Glossary |
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======== |
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|
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preemption |
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Prior to 2.5, or when ``CONFIG_PREEMPT`` is unset, processes in user |
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context inside the kernel would not preempt each other (ie. you had that |
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CPU until you gave it up, except for interrupts). With the addition of |
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``CONFIG_PREEMPT`` in 2.5.4, this changed: when in user context, higher |
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priority tasks can "cut in": spinlocks were changed to disable |
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preemption, even on UP. |
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|
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bh |
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Bottom Half: for historical reasons, functions with '_bh' in them often |
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now refer to any software interrupt, e.g. spin_lock_bh() |
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blocks any software interrupt on the current CPU. Bottom halves are |
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deprecated, and will eventually be replaced by tasklets. Only one bottom |
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half will be running at any time. |
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|
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Hardware Interrupt / Hardware IRQ |
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Hardware interrupt request. in_irq() returns true in a |
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hardware interrupt handler. |
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|
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Interrupt Context |
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Not user context: processing a hardware irq or software irq. Indicated |
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by the in_interrupt() macro returning true. |
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|
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SMP |
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Symmetric Multi-Processor: kernels compiled for multiple-CPU machines. |
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(``CONFIG_SMP=y``). |
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|
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Software Interrupt / softirq |
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Software interrupt handler. in_irq() returns false; |
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in_softirq() returns true. Tasklets and softirqs both |
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fall into the category of 'software interrupts'. |
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|
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Strictly speaking a softirq is one of up to 32 enumerated software |
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interrupts which can run on multiple CPUs at once. Sometimes used to |
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refer to tasklets as well (ie. all software interrupts). |
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|
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tasklet |
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A dynamically-registrable software interrupt, which is guaranteed to |
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only run on one CPU at a time. |
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|
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timer |
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A dynamically-registrable software interrupt, which is run at (or close |
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to) a given time. When running, it is just like a tasklet (in fact, they |
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are called from the ``TIMER_SOFTIRQ``). |
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UP |
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Uni-Processor: Non-SMP. (``CONFIG_SMP=n``). |
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|
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User Context |
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The kernel executing on behalf of a particular process (ie. a system |
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call or trap) or kernel thread. You can tell which process with the |
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``current`` macro.) Not to be confused with userspace. Can be |
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interrupted by software or hardware interrupts. |
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|
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Userspace |
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A process executing its own code outside the kernel.
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