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

[Dmitry Vyukov]

On Wed, Nov 4, 2015 at 5:01 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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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