Re: [PATCH -v3 7/8] locking: Move smp_cond_load_acquire() and friends into asm-generic/barrier.h

From: Will Deacon
Date: Wed Jun 01 2016 - 10:07:07 EST

On Wed, Jun 01, 2016 at 02:45:41PM +0200, Peter Zijlstra wrote:
> On Wed, Jun 01, 2016 at 01:13:33PM +0100, Will Deacon wrote:
> > On Wed, Jun 01, 2016 at 02:06:54PM +0200, Peter Zijlstra wrote:
> > > Works for me; but that would loose using cmpwait() for
> > > !smp_cond_load_acquire() spins, you fine with that?
> > >
> > > The two conversions in the patch were both !acquire spins.
> >
> > Maybe we could go the whole hog and add smp_cond_load_relaxed?
> What about say the cmpxchg loops in queued_write_lock_slowpath()
> ? Would that be something you'd like to use wfe for?

Without actually running the code on real hardware, it's hard to say
for sure. I notice that those loops are using cpu_relax_lowlatency
at present and we *know* that we're next in the queue (i.e. we're just
waiting for existing readers to drain), so the benefit of wfe is somewhat
questionable here and I don't think we'd want to add that initially.