lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
Open Source and information security mailing list archives
| ||
|
Date: Sat, 12 Oct 2019 07:49:58 +0200 From: Manfred Spraul <manfred@...orfullife.com> To: LKML <linux-kernel@...r.kernel.org>, Davidlohr Bueso <dave@...olabs.net>, Waiman Long <longman@...hat.com> Cc: 1vier1@....de, Andrew Morton <akpm@...ux-foundation.org>, Peter Zijlstra <peterz@...radead.org>, Jonathan Corbet <corbet@....net>, Manfred Spraul <manfred@...orfullife.com> Subject: [PATCH 6/6] Documentation/memory-barriers.txt: Clarify cmpxchg() The documentation in memory-barriers.txt claims that smp_mb__{before,after}_atomic() are for atomic ops that do not return a value. This is misleading and doesn't match the example in atomic_t.txt, and e.g. smp_mb__before_atomic() may and is used together with cmpxchg_relaxed() in the wake_q code. The purpose of e.g. smp_mb__before_atomic() is to "upgrade" a following RMW atomic operation to a full memory barrier. The return code of the atomic operation has no impact, so all of the following examples are valid: 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(); Signed-off-by: Manfred Spraul <manfred@...orfullife.com> Cc: Waiman Long <longman@...hat.com> Cc: Davidlohr Bueso <dave@...olabs.net> Cc: Peter Zijlstra <peterz@...radead.org> --- Documentation/memory-barriers.txt | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-) diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 1adbb8a371c7..52076b057400 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -1873,12 +1873,13 @@ 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 (such as add, subtract, + increment, decrement, failed conditional operations, ...) that do + not imply memory barriers, but where the code needs a memory barrier, + for example when 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 imply a full + 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: -- 2.21.0
Powered by blists - more mailing lists