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469 lines
17 KiB
469 lines
17 KiB
.. _rcu_dereference_doc: |
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PROPER CARE AND FEEDING OF RETURN VALUES FROM rcu_dereference() |
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=============================================================== |
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Most of the time, you can use values from rcu_dereference() or one of |
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the similar primitives without worries. Dereferencing (prefix "*"), |
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field selection ("->"), assignment ("="), address-of ("&"), addition and |
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subtraction of constants, and casts all work quite naturally and safely. |
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It is nevertheless possible to get into trouble with other operations. |
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Follow these rules to keep your RCU code working properly: |
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- You must use one of the rcu_dereference() family of primitives |
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to load an RCU-protected pointer, otherwise CONFIG_PROVE_RCU |
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will complain. Worse yet, your code can see random memory-corruption |
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bugs due to games that compilers and DEC Alpha can play. |
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Without one of the rcu_dereference() primitives, compilers |
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can reload the value, and won't your code have fun with two |
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different values for a single pointer! Without rcu_dereference(), |
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DEC Alpha can load a pointer, dereference that pointer, and |
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return data preceding initialization that preceded the store of |
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the pointer. |
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In addition, the volatile cast in rcu_dereference() prevents the |
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compiler from deducing the resulting pointer value. Please see |
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the section entitled "EXAMPLE WHERE THE COMPILER KNOWS TOO MUCH" |
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for an example where the compiler can in fact deduce the exact |
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value of the pointer, and thus cause misordering. |
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- In the special case where data is added but is never removed |
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while readers are accessing the structure, READ_ONCE() may be used |
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instead of rcu_dereference(). In this case, use of READ_ONCE() |
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takes on the role of the lockless_dereference() primitive that |
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was removed in v4.15. |
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- You are only permitted to use rcu_dereference on pointer values. |
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The compiler simply knows too much about integral values to |
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trust it to carry dependencies through integer operations. |
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There are a very few exceptions, namely that you can temporarily |
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cast the pointer to uintptr_t in order to: |
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- Set bits and clear bits down in the must-be-zero low-order |
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bits of that pointer. This clearly means that the pointer |
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must have alignment constraints, for example, this does |
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*not* work in general for char* pointers. |
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- XOR bits to translate pointers, as is done in some |
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classic buddy-allocator algorithms. |
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It is important to cast the value back to pointer before |
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doing much of anything else with it. |
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- Avoid cancellation when using the "+" and "-" infix arithmetic |
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operators. For example, for a given variable "x", avoid |
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"(x-(uintptr_t)x)" for char* pointers. The compiler is within its |
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rights to substitute zero for this sort of expression, so that |
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subsequent accesses no longer depend on the rcu_dereference(), |
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again possibly resulting in bugs due to misordering. |
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Of course, if "p" is a pointer from rcu_dereference(), and "a" |
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and "b" are integers that happen to be equal, the expression |
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"p+a-b" is safe because its value still necessarily depends on |
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the rcu_dereference(), thus maintaining proper ordering. |
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- If you are using RCU to protect JITed functions, so that the |
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"()" function-invocation operator is applied to a value obtained |
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(directly or indirectly) from rcu_dereference(), you may need to |
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interact directly with the hardware to flush instruction caches. |
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This issue arises on some systems when a newly JITed function is |
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using the same memory that was used by an earlier JITed function. |
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- Do not use the results from relational operators ("==", "!=", |
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">", ">=", "<", or "<=") when dereferencing. For example, |
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the following (quite strange) code is buggy:: |
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int *p; |
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int *q; |
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... |
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p = rcu_dereference(gp) |
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q = &global_q; |
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q += p > &oom_p; |
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r1 = *q; /* BUGGY!!! */ |
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As before, the reason this is buggy is that relational operators |
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are often compiled using branches. And as before, although |
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weak-memory machines such as ARM or PowerPC do order stores |
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after such branches, but can speculate loads, which can again |
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result in misordering bugs. |
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- Be very careful about comparing pointers obtained from |
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rcu_dereference() against non-NULL values. As Linus Torvalds |
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explained, if the two pointers are equal, the compiler could |
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substitute the pointer you are comparing against for the pointer |
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obtained from rcu_dereference(). For example:: |
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p = rcu_dereference(gp); |
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if (p == &default_struct) |
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do_default(p->a); |
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Because the compiler now knows that the value of "p" is exactly |
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the address of the variable "default_struct", it is free to |
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transform this code into the following:: |
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p = rcu_dereference(gp); |
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if (p == &default_struct) |
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do_default(default_struct.a); |
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On ARM and Power hardware, the load from "default_struct.a" |
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can now be speculated, such that it might happen before the |
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rcu_dereference(). This could result in bugs due to misordering. |
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However, comparisons are OK in the following cases: |
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- The comparison was against the NULL pointer. If the |
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compiler knows that the pointer is NULL, you had better |
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not be dereferencing it anyway. If the comparison is |
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non-equal, the compiler is none the wiser. Therefore, |
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it is safe to compare pointers from rcu_dereference() |
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against NULL pointers. |
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- The pointer is never dereferenced after being compared. |
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Since there are no subsequent dereferences, the compiler |
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cannot use anything it learned from the comparison |
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to reorder the non-existent subsequent dereferences. |
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This sort of comparison occurs frequently when scanning |
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RCU-protected circular linked lists. |
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Note that if checks for being within an RCU read-side |
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critical section are not required and the pointer is never |
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dereferenced, rcu_access_pointer() should be used in place |
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of rcu_dereference(). |
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- The comparison is against a pointer that references memory |
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that was initialized "a long time ago." The reason |
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this is safe is that even if misordering occurs, the |
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misordering will not affect the accesses that follow |
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the comparison. So exactly how long ago is "a long |
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time ago"? Here are some possibilities: |
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- Compile time. |
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- Boot time. |
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- Module-init time for module code. |
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- Prior to kthread creation for kthread code. |
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- During some prior acquisition of the lock that |
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we now hold. |
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- Before mod_timer() time for a timer handler. |
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There are many other possibilities involving the Linux |
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kernel's wide array of primitives that cause code to |
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be invoked at a later time. |
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- The pointer being compared against also came from |
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rcu_dereference(). In this case, both pointers depend |
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on one rcu_dereference() or another, so you get proper |
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ordering either way. |
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That said, this situation can make certain RCU usage |
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bugs more likely to happen. Which can be a good thing, |
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at least if they happen during testing. An example |
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of such an RCU usage bug is shown in the section titled |
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"EXAMPLE OF AMPLIFIED RCU-USAGE BUG". |
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- All of the accesses following the comparison are stores, |
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so that a control dependency preserves the needed ordering. |
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That said, it is easy to get control dependencies wrong. |
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Please see the "CONTROL DEPENDENCIES" section of |
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Documentation/memory-barriers.txt for more details. |
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- The pointers are not equal *and* the compiler does |
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not have enough information to deduce the value of the |
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pointer. Note that the volatile cast in rcu_dereference() |
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will normally prevent the compiler from knowing too much. |
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However, please note that if the compiler knows that the |
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pointer takes on only one of two values, a not-equal |
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comparison will provide exactly the information that the |
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compiler needs to deduce the value of the pointer. |
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- Disable any value-speculation optimizations that your compiler |
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might provide, especially if you are making use of feedback-based |
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optimizations that take data collected from prior runs. Such |
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value-speculation optimizations reorder operations by design. |
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There is one exception to this rule: Value-speculation |
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optimizations that leverage the branch-prediction hardware are |
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safe on strongly ordered systems (such as x86), but not on weakly |
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ordered systems (such as ARM or Power). Choose your compiler |
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command-line options wisely! |
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EXAMPLE OF AMPLIFIED RCU-USAGE BUG |
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---------------------------------- |
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Because updaters can run concurrently with RCU readers, RCU readers can |
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see stale and/or inconsistent values. If RCU readers need fresh or |
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consistent values, which they sometimes do, they need to take proper |
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precautions. To see this, consider the following code fragment:: |
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struct foo { |
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int a; |
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int b; |
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int c; |
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}; |
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struct foo *gp1; |
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struct foo *gp2; |
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void updater(void) |
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{ |
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struct foo *p; |
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p = kmalloc(...); |
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if (p == NULL) |
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deal_with_it(); |
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p->a = 42; /* Each field in its own cache line. */ |
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p->b = 43; |
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p->c = 44; |
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rcu_assign_pointer(gp1, p); |
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p->b = 143; |
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p->c = 144; |
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rcu_assign_pointer(gp2, p); |
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} |
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void reader(void) |
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{ |
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struct foo *p; |
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struct foo *q; |
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int r1, r2; |
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p = rcu_dereference(gp2); |
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if (p == NULL) |
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return; |
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r1 = p->b; /* Guaranteed to get 143. */ |
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q = rcu_dereference(gp1); /* Guaranteed non-NULL. */ |
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if (p == q) { |
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/* The compiler decides that q->c is same as p->c. */ |
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r2 = p->c; /* Could get 44 on weakly order system. */ |
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} |
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do_something_with(r1, r2); |
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} |
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You might be surprised that the outcome (r1 == 143 && r2 == 44) is possible, |
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but you should not be. After all, the updater might have been invoked |
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a second time between the time reader() loaded into "r1" and the time |
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that it loaded into "r2". The fact that this same result can occur due |
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to some reordering from the compiler and CPUs is beside the point. |
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But suppose that the reader needs a consistent view? |
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Then one approach is to use locking, for example, as follows:: |
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struct foo { |
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int a; |
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int b; |
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int c; |
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spinlock_t lock; |
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}; |
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struct foo *gp1; |
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struct foo *gp2; |
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void updater(void) |
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{ |
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struct foo *p; |
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p = kmalloc(...); |
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if (p == NULL) |
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deal_with_it(); |
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spin_lock(&p->lock); |
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p->a = 42; /* Each field in its own cache line. */ |
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p->b = 43; |
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p->c = 44; |
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spin_unlock(&p->lock); |
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rcu_assign_pointer(gp1, p); |
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spin_lock(&p->lock); |
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p->b = 143; |
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p->c = 144; |
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spin_unlock(&p->lock); |
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rcu_assign_pointer(gp2, p); |
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} |
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void reader(void) |
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{ |
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struct foo *p; |
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struct foo *q; |
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int r1, r2; |
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p = rcu_dereference(gp2); |
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if (p == NULL) |
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return; |
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spin_lock(&p->lock); |
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r1 = p->b; /* Guaranteed to get 143. */ |
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q = rcu_dereference(gp1); /* Guaranteed non-NULL. */ |
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if (p == q) { |
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/* The compiler decides that q->c is same as p->c. */ |
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r2 = p->c; /* Locking guarantees r2 == 144. */ |
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} |
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spin_unlock(&p->lock); |
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do_something_with(r1, r2); |
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} |
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As always, use the right tool for the job! |
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EXAMPLE WHERE THE COMPILER KNOWS TOO MUCH |
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----------------------------------------- |
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If a pointer obtained from rcu_dereference() compares not-equal to some |
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other pointer, the compiler normally has no clue what the value of the |
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first pointer might be. This lack of knowledge prevents the compiler |
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from carrying out optimizations that otherwise might destroy the ordering |
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guarantees that RCU depends on. And the volatile cast in rcu_dereference() |
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should prevent the compiler from guessing the value. |
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But without rcu_dereference(), the compiler knows more than you might |
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expect. Consider the following code fragment:: |
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struct foo { |
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int a; |
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int b; |
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}; |
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static struct foo variable1; |
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static struct foo variable2; |
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static struct foo *gp = &variable1; |
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void updater(void) |
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{ |
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initialize_foo(&variable2); |
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rcu_assign_pointer(gp, &variable2); |
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/* |
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* The above is the only store to gp in this translation unit, |
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* and the address of gp is not exported in any way. |
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*/ |
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} |
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int reader(void) |
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{ |
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struct foo *p; |
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p = gp; |
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barrier(); |
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if (p == &variable1) |
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return p->a; /* Must be variable1.a. */ |
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else |
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return p->b; /* Must be variable2.b. */ |
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} |
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Because the compiler can see all stores to "gp", it knows that the only |
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possible values of "gp" are "variable1" on the one hand and "variable2" |
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on the other. The comparison in reader() therefore tells the compiler |
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the exact value of "p" even in the not-equals case. This allows the |
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compiler to make the return values independent of the load from "gp", |
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in turn destroying the ordering between this load and the loads of the |
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return values. This can result in "p->b" returning pre-initialization |
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garbage values. |
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In short, rcu_dereference() is *not* optional when you are going to |
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dereference the resulting pointer. |
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WHICH MEMBER OF THE rcu_dereference() FAMILY SHOULD YOU USE? |
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------------------------------------------------------------ |
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First, please avoid using rcu_dereference_raw() and also please avoid |
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using rcu_dereference_check() and rcu_dereference_protected() with a |
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second argument with a constant value of 1 (or true, for that matter). |
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With that caution out of the way, here is some guidance for which |
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member of the rcu_dereference() to use in various situations: |
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1. If the access needs to be within an RCU read-side critical |
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section, use rcu_dereference(). With the new consolidated |
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RCU flavors, an RCU read-side critical section is entered |
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using rcu_read_lock(), anything that disables bottom halves, |
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anything that disables interrupts, or anything that disables |
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preemption. |
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2. If the access might be within an RCU read-side critical section |
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on the one hand, or protected by (say) my_lock on the other, |
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use rcu_dereference_check(), for example:: |
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p1 = rcu_dereference_check(p->rcu_protected_pointer, |
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lockdep_is_held(&my_lock)); |
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3. If the access might be within an RCU read-side critical section |
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on the one hand, or protected by either my_lock or your_lock on |
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the other, again use rcu_dereference_check(), for example:: |
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p1 = rcu_dereference_check(p->rcu_protected_pointer, |
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lockdep_is_held(&my_lock) || |
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lockdep_is_held(&your_lock)); |
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4. If the access is on the update side, so that it is always protected |
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by my_lock, use rcu_dereference_protected():: |
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p1 = rcu_dereference_protected(p->rcu_protected_pointer, |
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lockdep_is_held(&my_lock)); |
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This can be extended to handle multiple locks as in #3 above, |
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and both can be extended to check other conditions as well. |
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5. If the protection is supplied by the caller, and is thus unknown |
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to this code, that is the rare case when rcu_dereference_raw() |
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is appropriate. In addition, rcu_dereference_raw() might be |
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appropriate when the lockdep expression would be excessively |
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complex, except that a better approach in that case might be to |
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take a long hard look at your synchronization design. Still, |
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there are data-locking cases where any one of a very large number |
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of locks or reference counters suffices to protect the pointer, |
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so rcu_dereference_raw() does have its place. |
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However, its place is probably quite a bit smaller than one |
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might expect given the number of uses in the current kernel. |
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Ditto for its synonym, rcu_dereference_check( ... , 1), and |
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its close relative, rcu_dereference_protected(... , 1). |
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SPARSE CHECKING OF RCU-PROTECTED POINTERS |
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----------------------------------------- |
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The sparse static-analysis tool checks for direct access to RCU-protected |
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pointers, which can result in "interesting" bugs due to compiler |
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optimizations involving invented loads and perhaps also load tearing. |
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For example, suppose someone mistakenly does something like this:: |
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p = q->rcu_protected_pointer; |
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do_something_with(p->a); |
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do_something_else_with(p->b); |
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If register pressure is high, the compiler might optimize "p" out |
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of existence, transforming the code to something like this:: |
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do_something_with(q->rcu_protected_pointer->a); |
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do_something_else_with(q->rcu_protected_pointer->b); |
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This could fatally disappoint your code if q->rcu_protected_pointer |
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changed in the meantime. Nor is this a theoretical problem: Exactly |
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this sort of bug cost Paul E. McKenney (and several of his innocent |
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colleagues) a three-day weekend back in the early 1990s. |
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Load tearing could of course result in dereferencing a mashup of a pair |
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of pointers, which also might fatally disappoint your code. |
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These problems could have been avoided simply by making the code instead |
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read as follows:: |
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p = rcu_dereference(q->rcu_protected_pointer); |
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do_something_with(p->a); |
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do_something_else_with(p->b); |
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Unfortunately, these sorts of bugs can be extremely hard to spot during |
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review. This is where the sparse tool comes into play, along with the |
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"__rcu" marker. If you mark a pointer declaration, whether in a structure |
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or as a formal parameter, with "__rcu", which tells sparse to complain if |
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this pointer is accessed directly. It will also cause sparse to complain |
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if a pointer not marked with "__rcu" is accessed using rcu_dereference() |
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and friends. For example, ->rcu_protected_pointer might be declared as |
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follows:: |
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struct foo __rcu *rcu_protected_pointer; |
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Use of "__rcu" is opt-in. If you choose not to use it, then you should |
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ignore the sparse warnings.
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