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355 lines
11 KiB
355 lines
11 KiB
/* SPDX-License-Identifier: GPL-2.0 */ |
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/* |
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* Percpu refcounts: |
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* (C) 2012 Google, Inc. |
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* Author: Kent Overstreet <[email protected]> |
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* |
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* This implements a refcount with similar semantics to atomic_t - atomic_inc(), |
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* atomic_dec_and_test() - but percpu. |
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* |
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* There's one important difference between percpu refs and normal atomic_t |
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* refcounts; you have to keep track of your initial refcount, and then when you |
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* start shutting down you call percpu_ref_kill() _before_ dropping the initial |
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* refcount. |
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* |
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* The refcount will have a range of 0 to ((1U << 31) - 1), i.e. one bit less |
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* than an atomic_t - this is because of the way shutdown works, see |
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* percpu_ref_kill()/PERCPU_COUNT_BIAS. |
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* |
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* Before you call percpu_ref_kill(), percpu_ref_put() does not check for the |
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* refcount hitting 0 - it can't, if it was in percpu mode. percpu_ref_kill() |
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* puts the ref back in single atomic_t mode, collecting the per cpu refs and |
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* issuing the appropriate barriers, and then marks the ref as shutting down so |
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* that percpu_ref_put() will check for the ref hitting 0. After it returns, |
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* it's safe to drop the initial ref. |
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* |
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* USAGE: |
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* |
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* See fs/aio.c for some example usage; it's used there for struct kioctx, which |
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* is created when userspaces calls io_setup(), and destroyed when userspace |
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* calls io_destroy() or the process exits. |
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* |
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* In the aio code, kill_ioctx() is called when we wish to destroy a kioctx; it |
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* removes the kioctx from the proccess's table of kioctxs and kills percpu_ref. |
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* After that, there can't be any new users of the kioctx (from lookup_ioctx()) |
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* and it's then safe to drop the initial ref with percpu_ref_put(). |
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* |
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* Note that the free path, free_ioctx(), needs to go through explicit call_rcu() |
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* to synchronize with RCU protected lookup_ioctx(). percpu_ref operations don't |
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* imply RCU grace periods of any kind and if a user wants to combine percpu_ref |
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* with RCU protection, it must be done explicitly. |
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* |
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* Code that does a two stage shutdown like this often needs some kind of |
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* explicit synchronization to ensure the initial refcount can only be dropped |
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* once - percpu_ref_kill() does this for you, it returns true once and false if |
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* someone else already called it. The aio code uses it this way, but it's not |
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* necessary if the code has some other mechanism to synchronize teardown. |
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* around. |
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*/ |
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#ifndef _LINUX_PERCPU_REFCOUNT_H |
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#define _LINUX_PERCPU_REFCOUNT_H |
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#include <linux/atomic.h> |
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#include <linux/kernel.h> |
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#include <linux/percpu.h> |
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#include <linux/rcupdate.h> |
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#include <linux/gfp.h> |
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struct percpu_ref; |
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typedef void (percpu_ref_func_t)(struct percpu_ref *); |
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/* flags set in the lower bits of percpu_ref->percpu_count_ptr */ |
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enum { |
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__PERCPU_REF_ATOMIC = 1LU << 0, /* operating in atomic mode */ |
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__PERCPU_REF_DEAD = 1LU << 1, /* (being) killed */ |
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__PERCPU_REF_ATOMIC_DEAD = __PERCPU_REF_ATOMIC | __PERCPU_REF_DEAD, |
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__PERCPU_REF_FLAG_BITS = 2, |
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}; |
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/* @flags for percpu_ref_init() */ |
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enum { |
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/* |
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* Start w/ ref == 1 in atomic mode. Can be switched to percpu |
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* operation using percpu_ref_switch_to_percpu(). If initialized |
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* with this flag, the ref will stay in atomic mode until |
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* percpu_ref_switch_to_percpu() is invoked on it. |
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* Implies ALLOW_REINIT. |
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*/ |
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PERCPU_REF_INIT_ATOMIC = 1 << 0, |
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/* |
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* Start dead w/ ref == 0 in atomic mode. Must be revived with |
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* percpu_ref_reinit() before used. Implies INIT_ATOMIC and |
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* ALLOW_REINIT. |
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*/ |
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PERCPU_REF_INIT_DEAD = 1 << 1, |
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/* |
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* Allow switching from atomic mode to percpu mode. |
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*/ |
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PERCPU_REF_ALLOW_REINIT = 1 << 2, |
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}; |
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struct percpu_ref_data { |
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atomic_long_t count; |
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percpu_ref_func_t *release; |
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percpu_ref_func_t *confirm_switch; |
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bool force_atomic:1; |
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bool allow_reinit:1; |
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struct rcu_head rcu; |
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struct percpu_ref *ref; |
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}; |
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struct percpu_ref { |
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/* |
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* The low bit of the pointer indicates whether the ref is in percpu |
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* mode; if set, then get/put will manipulate the atomic_t. |
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*/ |
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unsigned long percpu_count_ptr; |
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/* |
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* 'percpu_ref' is often embedded into user structure, and only |
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* 'percpu_count_ptr' is required in fast path, move other fields |
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* into 'percpu_ref_data', so we can reduce memory footprint in |
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* fast path. |
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*/ |
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struct percpu_ref_data *data; |
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}; |
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int __must_check percpu_ref_init(struct percpu_ref *ref, |
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percpu_ref_func_t *release, unsigned int flags, |
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gfp_t gfp); |
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void percpu_ref_exit(struct percpu_ref *ref); |
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void percpu_ref_switch_to_atomic(struct percpu_ref *ref, |
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percpu_ref_func_t *confirm_switch); |
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void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref); |
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void percpu_ref_switch_to_percpu(struct percpu_ref *ref); |
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void percpu_ref_kill_and_confirm(struct percpu_ref *ref, |
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percpu_ref_func_t *confirm_kill); |
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void percpu_ref_resurrect(struct percpu_ref *ref); |
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void percpu_ref_reinit(struct percpu_ref *ref); |
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bool percpu_ref_is_zero(struct percpu_ref *ref); |
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/** |
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* percpu_ref_kill - drop the initial ref |
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* @ref: percpu_ref to kill |
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* |
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* Must be used to drop the initial ref on a percpu refcount; must be called |
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* precisely once before shutdown. |
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* |
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* Switches @ref into atomic mode before gathering up the percpu counters |
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* and dropping the initial ref. |
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* |
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* There are no implied RCU grace periods between kill and release. |
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*/ |
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static inline void percpu_ref_kill(struct percpu_ref *ref) |
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{ |
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percpu_ref_kill_and_confirm(ref, NULL); |
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} |
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/* |
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* Internal helper. Don't use outside percpu-refcount proper. The |
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* function doesn't return the pointer and let the caller test it for NULL |
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* because doing so forces the compiler to generate two conditional |
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* branches as it can't assume that @ref->percpu_count is not NULL. |
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*/ |
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static inline bool __ref_is_percpu(struct percpu_ref *ref, |
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unsigned long __percpu **percpu_countp) |
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{ |
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unsigned long percpu_ptr; |
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/* |
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* The value of @ref->percpu_count_ptr is tested for |
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* !__PERCPU_REF_ATOMIC, which may be set asynchronously, and then |
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* used as a pointer. If the compiler generates a separate fetch |
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* when using it as a pointer, __PERCPU_REF_ATOMIC may be set in |
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* between contaminating the pointer value, meaning that |
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* READ_ONCE() is required when fetching it. |
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* |
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* The dependency ordering from the READ_ONCE() pairs |
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* with smp_store_release() in __percpu_ref_switch_to_percpu(). |
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*/ |
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percpu_ptr = READ_ONCE(ref->percpu_count_ptr); |
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/* |
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* Theoretically, the following could test just ATOMIC; however, |
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* then we'd have to mask off DEAD separately as DEAD may be |
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* visible without ATOMIC if we race with percpu_ref_kill(). DEAD |
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* implies ATOMIC anyway. Test them together. |
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*/ |
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if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD)) |
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return false; |
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*percpu_countp = (unsigned long __percpu *)percpu_ptr; |
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return true; |
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} |
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/** |
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* percpu_ref_get_many - increment a percpu refcount |
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* @ref: percpu_ref to get |
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* @nr: number of references to get |
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* |
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* Analogous to atomic_long_add(). |
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* |
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* This function is safe to call as long as @ref is between init and exit. |
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*/ |
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static inline void percpu_ref_get_many(struct percpu_ref *ref, unsigned long nr) |
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{ |
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unsigned long __percpu *percpu_count; |
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rcu_read_lock(); |
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if (__ref_is_percpu(ref, &percpu_count)) |
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this_cpu_add(*percpu_count, nr); |
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else |
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atomic_long_add(nr, &ref->data->count); |
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rcu_read_unlock(); |
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} |
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/** |
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* percpu_ref_get - increment a percpu refcount |
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* @ref: percpu_ref to get |
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* |
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* Analagous to atomic_long_inc(). |
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* |
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* This function is safe to call as long as @ref is between init and exit. |
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*/ |
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static inline void percpu_ref_get(struct percpu_ref *ref) |
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{ |
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percpu_ref_get_many(ref, 1); |
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} |
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/** |
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* percpu_ref_tryget_many - try to increment a percpu refcount |
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* @ref: percpu_ref to try-get |
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* @nr: number of references to get |
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* |
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* Increment a percpu refcount by @nr unless its count already reached zero. |
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* Returns %true on success; %false on failure. |
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* |
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* This function is safe to call as long as @ref is between init and exit. |
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*/ |
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static inline bool percpu_ref_tryget_many(struct percpu_ref *ref, |
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unsigned long nr) |
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{ |
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unsigned long __percpu *percpu_count; |
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bool ret; |
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rcu_read_lock(); |
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if (__ref_is_percpu(ref, &percpu_count)) { |
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this_cpu_add(*percpu_count, nr); |
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ret = true; |
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} else { |
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ret = atomic_long_add_unless(&ref->data->count, nr, 0); |
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} |
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rcu_read_unlock(); |
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return ret; |
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} |
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/** |
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* percpu_ref_tryget - try to increment a percpu refcount |
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* @ref: percpu_ref to try-get |
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* |
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* Increment a percpu refcount unless its count already reached zero. |
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* Returns %true on success; %false on failure. |
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* |
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* This function is safe to call as long as @ref is between init and exit. |
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*/ |
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static inline bool percpu_ref_tryget(struct percpu_ref *ref) |
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{ |
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return percpu_ref_tryget_many(ref, 1); |
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} |
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/** |
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* percpu_ref_tryget_live - try to increment a live percpu refcount |
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* @ref: percpu_ref to try-get |
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* |
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* Increment a percpu refcount unless it has already been killed. Returns |
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* %true on success; %false on failure. |
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* |
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* Completion of percpu_ref_kill() in itself doesn't guarantee that this |
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* function will fail. For such guarantee, percpu_ref_kill_and_confirm() |
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* should be used. After the confirm_kill callback is invoked, it's |
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* guaranteed that no new reference will be given out by |
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* percpu_ref_tryget_live(). |
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* |
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* This function is safe to call as long as @ref is between init and exit. |
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*/ |
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static inline bool percpu_ref_tryget_live(struct percpu_ref *ref) |
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{ |
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unsigned long __percpu *percpu_count; |
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bool ret = false; |
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rcu_read_lock(); |
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if (__ref_is_percpu(ref, &percpu_count)) { |
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this_cpu_inc(*percpu_count); |
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ret = true; |
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} else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) { |
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ret = atomic_long_inc_not_zero(&ref->data->count); |
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} |
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rcu_read_unlock(); |
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return ret; |
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} |
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/** |
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* percpu_ref_put_many - decrement a percpu refcount |
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* @ref: percpu_ref to put |
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* @nr: number of references to put |
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* |
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* Decrement the refcount, and if 0, call the release function (which was passed |
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* to percpu_ref_init()) |
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* |
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* This function is safe to call as long as @ref is between init and exit. |
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*/ |
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static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr) |
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{ |
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unsigned long __percpu *percpu_count; |
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rcu_read_lock(); |
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if (__ref_is_percpu(ref, &percpu_count)) |
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this_cpu_sub(*percpu_count, nr); |
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else if (unlikely(atomic_long_sub_and_test(nr, &ref->data->count))) |
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ref->data->release(ref); |
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rcu_read_unlock(); |
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} |
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/** |
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* percpu_ref_put - decrement a percpu refcount |
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* @ref: percpu_ref to put |
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* |
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* Decrement the refcount, and if 0, call the release function (which was passed |
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* to percpu_ref_init()) |
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* |
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* This function is safe to call as long as @ref is between init and exit. |
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*/ |
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static inline void percpu_ref_put(struct percpu_ref *ref) |
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{ |
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percpu_ref_put_many(ref, 1); |
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} |
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/** |
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* percpu_ref_is_dying - test whether a percpu refcount is dying or dead |
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* @ref: percpu_ref to test |
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* |
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* Returns %true if @ref is dying or dead. |
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* |
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* This function is safe to call as long as @ref is between init and exit |
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* and the caller is responsible for synchronizing against state changes. |
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*/ |
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static inline bool percpu_ref_is_dying(struct percpu_ref *ref) |
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{ |
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return ref->percpu_count_ptr & __PERCPU_REF_DEAD; |
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} |
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#endif
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