Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier

Peter Zijlstra
2016-06-15T18:40:07Z

On Wed, Jun 15, 2016 at 11:27:24AM -0700, Davidlohr Bueso wrote:
> On Wed, 15 Jun 2016, Peter Zijlstra wrote:
>
> >In any case, its fairly simple to cure, just add
> >smp_acquire__after_ctrl_dep() at the end. If we bail because
> >need_resched() we don't need the acquire I think.
>
> I was just considering this for your smp_cond_acquire/smp_cond_load_acquire

Right, so that need_resched break makes that a bit awkward. Not to
mention the cpu_relaxed() vs cpu_relaxed_lowlatency() difference.

> rework, so yeah I guess an smp_acquire__after_ctrl_dep would be a nice
> compromise.
>
> However, I was always under the impression that races with node->locked were
> rather harmless (as indicated in the mentioned commit) -- which is why ->locked
> are simple load/stores, with the exception of the unqueueing -- but yeah, that's
> not even paired.

Yeah, see a few patches further in this series, where he guards a
variables with the osq_lock.