Re: [PATCH] doc: Update wake_up() & co. memory-barrier guarantees

Andrea Parri
2018-06-25T10:56:33Z

On Mon, Jun 25, 2018 at 11:50:31AM +0200, Peter Zijlstra wrote:
> On Mon, Jun 25, 2018 at 11:17:38AM +0200, Andrea Parri wrote:
> > Both the implementation and the users' expectation [1] for the various
> > wakeup primitives have evolved over time, but the documentation has not
> > kept up with these changes: brings it into 2018.
>
> I wanted to reply to this saying that I'm not aware of anything relying
> on this actually being a smp_mb() and that I've been treating it as an
> RELEASE.
>
> But then I found my own comment that goes with smp_mb__after_spinlock(),
> which explains why we do in fact need the transitive thing if I'm not
> mistaken.

A concrete example being the store-buffering pattern reported in [1].

>
> So yes, I suppose we're entirely suck with the full memory barrier
> semantics like that. But I still find it easier to think of it like a
> RELEASE that pairs with the ACQUIRE of waking up, such that the task
> is guaranteed to observe it's own wake condition.
>
> And maybe that is the thing I'm missing here. These comments only state
> that it does in fact imply a full memory barrier, but do not explain
> why, should it?

"code (people) is relying on it" is really the only "why" I can think
of. With this patch, that same/SB pattern is also reported in memory
-barriers.txt. Other ideas?

Andrea