From 39323c64b8a95d10ddc66dc815dd14efdddf6777 Mon Sep 17 00:00:00 2001 From: Manfred Spraul Date: Mon, 3 Feb 2020 17:34:29 -0800 Subject: smp_mb__{before,after}_atomic(): update Documentation When adding the _{acquire|release|relaxed}() variants of some atomic operations, it was forgotten to update Documentation/memory_barrier.txt: smp_mb__{before,after}_atomic() is now intended for all RMW operations that do not imply a memory barrier. 1) smp_mb__before_atomic(); atomic_add(); 2) smp_mb__before_atomic(); atomic_xchg_relaxed(); 3) smp_mb__before_atomic(); atomic_fetch_add_relaxed(); Invalid would be: smp_mb__before_atomic(); atomic_set(); In addition, the patch splits the long sentence into multiple shorter sentences. Link: http://lkml.kernel.org/r/20191020123305.14715-2-manfred@colorfullife.com Fixes: 654672d4ba1a ("locking/atomics: Add _{acquire|release|relaxed}() variants of some atomic operations") Signed-off-by: Manfred Spraul Acked-by: Waiman Long Cc: Davidlohr Bueso Cc: Peter Zijlstra Cc: Will Deacon Cc: Peter Zijlstra Cc: <1vier1@web.de> Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- Documentation/memory-barriers.txt | 16 ++++++++++------ 1 file changed, 10 insertions(+), 6 deletions(-) (limited to 'Documentation/memory-barriers.txt') diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index ec3b5865c1be..7146da061693 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -1868,12 +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 also used for atomic bitop functions that do not return a - value (such as set_bit and clear_bit). + 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 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: -- cgit v1.2.3