linux-kernel - Re: [GIT PULL] locking changes for v4.4

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Message-ID: <CACT4Y+Z=26MrS23zH2fh2zE-v+HA4LtFpxgtwQfS4MqMJHWzcw@mail.gmail.com>
Date:	Wed, 4 Nov 2015 17:09:24 +0100
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] locking changes for v4.4

On Wed, Nov 4, 2015 at 5:01 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, Nov 04, 2015 at 03:51:01PM +0100, Dmitry Vyukov wrote:
>> To clarify, yes, documentation and tooling was my main motivation.
>
> Right; I don't object to having _ctrl() methods purely for documentation
> purposes, I keep finding places we rely on them. Having them stand out
> better might be useful.
>
>> It is usually helpful to see acquire/release, rmb/wmb pairs, and so it
>> is useful to know that something below is ordered wrt this load by
>> means of a control dependency (which effectively becomes an acquire,
>> and there must be a pairing release somewhere).
>
> You need at least a trailing smp_rmb() before you cover the ACQUIRE
> semantics -- or have no trailing reads at all of course.

Yes, I know, but is the best we can do. Episodic false negatives are
OK, while false positives are unacceptable. So we considered
READ_ONCE_CTRL as acquire.
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