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Diffstat (limited to 'Documentation/memory-barriers.txt')
| -rw-r--r-- | Documentation/memory-barriers.txt | 25 |
1 files changed, 12 insertions, 13 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 1adbb8a371c7..7146da061693 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -63,7 +63,6 @@ CONTENTS - Compiler barrier. - CPU memory barriers. - - MMIO write barrier. (*) Implicit kernel memory barriers. @@ -75,7 +74,6 @@ CONTENTS (*) Inter-CPU acquiring barrier effects. - Acquires vs memory accesses. - - Acquires vs I/O accesses. (*) Where are memory barriers needed? @@ -492,10 +490,9 @@ And a couple of implicit varieties: happen before it completes. The use of ACQUIRE and RELEASE operations generally precludes the need - for other sorts of memory barrier (but note the exceptions mentioned in - the subsection "MMIO write barrier"). In addition, a RELEASE+ACQUIRE - pair is -not- guaranteed to act as a full memory barrier. However, after - an ACQUIRE on a given variable, all memory accesses preceding any prior + for other sorts of memory barrier. In addition, a RELEASE+ACQUIRE pair is + -not- guaranteed to act as a full memory barrier. However, after an + ACQUIRE on a given variable, all memory accesses preceding any prior RELEASE on that same variable are guaranteed to be visible. In other words, within a given variable's critical section, all accesses of all previous critical sections for that variable are guaranteed to have @@ -1512,8 +1509,6 @@ levels: (*) CPU memory barriers. - (*) MMIO write barrier. - COMPILER BARRIER ---------------- @@ -1873,12 +1868,16 @@ There are some more advanced barrier functions: (*) smp_mb__before_atomic(); (*) smp_mb__after_atomic(); - These are for use with atomic (such as add, subtract, increment and - decrement) functions that don't return a value, especially when used for - reference counting. These functions do not imply memory barriers. + These are for use with atomic RMW functions that do not imply memory + barriers, but where the code needs a memory barrier. Examples for atomic + RMW functions that do not imply are memory barrier are e.g. add, + subtract, (failed) conditional operations, _relaxed functions, + but not atomic_read or atomic_set. A common example where a memory + barrier may be required is when atomic ops are used for reference + counting. - These are also used for atomic bitop functions that do not return a - value (such as set_bit and clear_bit). + These are also used for atomic RMW bitop functions that do not imply a + memory barrier (such as set_bit and clear_bit). As an example, consider a piece of code that marks an object as being dead and then decrements the object's reference count: |
