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Diffstat (limited to 'Documentation/memory-barriers.txt')
| -rw-r--r-- | Documentation/memory-barriers.txt | 68 |
1 files changed, 48 insertions, 20 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index f855031..92f0056 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -610,6 +610,7 @@ loads. Consider the following sequence of events: CPU 1 CPU 2 ======================= ======================= + { B = 7; X = 9; Y = 8; C = &Y } STORE A = 1 STORE B = 2 <write barrier> @@ -651,7 +652,20 @@ In the above example, CPU 2 perceives that B is 7, despite the load of *C (which would be B) coming after the the LOAD of C. If, however, a data dependency barrier were to be placed between the load of C -and the load of *C (ie: B) on CPU 2, then the following will occur: +and the load of *C (ie: B) on CPU 2: + + CPU 1 CPU 2 + ======================= ======================= + { B = 7; X = 9; Y = 8; C = &Y } + STORE A = 1 + STORE B = 2 + <write barrier> + STORE C = &B LOAD X + STORE D = 4 LOAD C (gets &B) + <data dependency barrier> + LOAD *C (reads B) + +then the following will occur: +-------+ : : : : | | +------+ +-------+ @@ -829,8 +843,8 @@ 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 +1277,17 @@ 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 +1298,31 @@ 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 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. +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. -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 +1343,12 @@ 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. |
