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

[Davidlohr Bueso]

On Wed, 15 Jun 2016, Peter Zijlstra wrote:

> 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.

Oh sure, I was merely refering to the ordering semantics, not the calls
themselves -- although at some point, as archs begin to port locking/core
optimizations, we _will_ need the variants for dealing with '_lowlatency'.

> > 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.

*sigh*