diff options
| -rw-r--r-- | Documentation/memory-barriers.txt | 52 |
1 files changed, 33 insertions, 19 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index f8550310a6d..528d52f52ee 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -829,8 +829,8 @@ There are some more advanced barrier functions: (*) smp_mb__after_atomic_inc(); These are for use with atomic add, subtract, increment and decrement - functions, especially when used for reference counting. These functions - do not imply memory barriers. + functions that don't return a value, especially when used for reference + counting. These functions do not imply memory barriers. As an example, consider a piece of code that marks an object as being dead and then decrements the object's reference count: @@ -1263,15 +1263,17 @@ else. ATOMIC OPERATIONS ----------------- -Though they are technically interprocessor interaction considerations, atomic -operations are noted specially as they do _not_ generally imply memory -barriers. The possible offenders include: +Whilst they are technically interprocessor interaction considerations, atomic +operations are noted specially as some of them imply full memory barriers and +some don't, but they're very heavily relied on as a group throughout the +kernel. + +Any atomic operation that modifies some state in memory and returns information +about the state (old or new) implies an SMP-conditional general memory barrier +(smp_mb()) on each side of the actual operation. These include: xchg(); cmpxchg(); - test_and_set_bit(); - test_and_clear_bit(); - test_and_change_bit(); atomic_cmpxchg(); atomic_inc_return(); atomic_dec_return(); @@ -1282,21 +1284,31 @@ barriers. The possible offenders include: atomic_sub_and_test(); atomic_add_negative(); atomic_add_unless(); + test_and_set_bit(); + test_and_clear_bit(); + test_and_change_bit(); + +These are used for such things as implementing LOCK-class and UNLOCK-class +operations and adjusting reference counters towards object destruction, and as +such the implicit memory barrier effects are necessary. -These may be used for such things as implementing LOCK operations or controlling -the lifetime of objects by decreasing their reference counts. In such cases -they need preceding memory barriers. -The following may also be possible offenders as they may be used as UNLOCK -operations. +The following operation are potential problems as they do _not_ imply memory +barriers, but might be used for implementing such things as UNLOCK-class +operations: + atomic_set(); set_bit(); clear_bit(); change_bit(); - atomic_set(); + +With these the appropriate explicit memory barrier should be used if necessary +(smp_mb__before_clear_bit() for instance). -The following are a little tricky: +The following also do _not_ imply memory barriers, and so may require explicit +memory barriers under some circumstances (smp_mb__before_atomic_dec() for +instance)): atomic_add(); atomic_sub(); @@ -1317,10 +1329,12 @@ specific order. Basically, each usage case has to be carefully considered as to whether memory -barriers are needed or not. The simplest rule is probably: if the atomic -operation is protected by a lock, then it does not require a barrier unless -there's another operation within the critical section with respect to which an -ordering must be maintained. +barriers are needed or not. + +[!] Note that special memory barrier primitives are available for these +situations because on some CPUs the atomic instructions used imply full memory +barriers, and so barrier instructions are superfluous in conjunction with them, +and in such cases the special barrier primitives will be no-ops. See Documentation/atomic_ops.txt for more information. |