linux-kernel - Re: [tip:locking/core] locking/Documentation: Clarify failed cmpxchg( ) memory ordering semantics

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Date:	Mon, 03 Aug 2015 10:36:53 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	will.deacon@....com, paulmck@...ux.vnet.ibm.com,
	peterz@...radead.org, mingo@...nel.org,
	linux-kernel@...r.kernel.org, doug.hatch@...com,
	torvalds@...ux-foundation.org, hpa@...or.com,
	akpm@...ux-foundation.org, corbet@....net, scott.norton@...com,
	waiman.long@...com, tglx@...utronix.de
Cc:	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:locking/core] locking/Documentation: Clarify failed
 cmpxchg( ) memory ordering semantics

On Mon, 2015-08-03 at 09:59 -0700, tip-bot for Will Deacon wrote:
> Commit-ID:  ed2de9f74ecbbf3063d29b2334e7b455d7f35189
> Gitweb:     http://git.kernel.org/tip/ed2de9f74ecbbf3063d29b2334e7b455d7f35189
> Author:     Will Deacon <will.deacon@....com>
> AuthorDate: Thu, 16 Jul 2015 16:10:06 +0100
> Committer:  Ingo Molnar <mingo@...nel.org>
> CommitDate: Mon, 3 Aug 2015 10:57:09 +0200
> 
> locking/Documentation: Clarify failed cmpxchg() memory ordering semantics
> 
> A failed cmpxchg does not provide any memory ordering guarantees, a
> property that is used to optimise the cmpxchg implementations on Alpha,
> PowerPC and arm64.
> 
> This patch updates atomic_ops.txt and memory-barriers.txt to reflect
> this.
> 
> Signed-off-by: Will Deacon <will.deacon@....com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Davidlohr Bueso <dave@...olabs.net>
> Cc: Douglas Hatch <doug.hatch@...com>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: Jonathan Corbet <corbet@....net>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Scott J Norton <scott.norton@...com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Waiman Long <waiman.long@...com>
> Link: http://lkml.kernel.org/r/20150716151006.GH26390@arm.com
> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> ---
>  Documentation/atomic_ops.txt      | 4 +++-
>  Documentation/memory-barriers.txt | 6 +++---
>  2 files changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/atomic_ops.txt b/Documentation/atomic_ops.txt
> index dab6da3..b19fc34 100644
> --- a/Documentation/atomic_ops.txt
> +++ b/Documentation/atomic_ops.txt
> @@ -266,7 +266,9 @@ with the given old and new values. Like all atomic_xxx operations,
>  atomic_cmpxchg will only satisfy its atomicity semantics as long as all
>  other accesses of *v are performed through atomic_xxx operations.
>  
> -atomic_cmpxchg must provide explicit memory barriers around the operation.
> +atomic_cmpxchg must provide explicit memory barriers around the operation,
> +although if the comparison fails then no memory ordering guarantees are
> +required.

Thanks, I also stumbled upon this with the wake-q stuff.

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