Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()

Paul E. McKenney
2015-11-12T15:20:40Z

On Thu, Nov 12, 2015 at 04:08:22PM +0100, Peter Zijlstra wrote:
> On Thu, Nov 12, 2015 at 07:01:49AM -0800, Paul E. McKenney wrote:
> > On Thu, Nov 12, 2015 at 03:50:13PM +0100, Peter Zijlstra wrote:
> > > On Thu, Nov 12, 2015 at 06:40:04AM -0800, Paul E. McKenney wrote:
> > > > I cannot resist suggesting that any lock that interacts with
> > > > spin_unlock_wait() must have all relevant acquisitions followed by
> > > > smp_mb__after_unlock_lock().
> > >
> > > Ha! that would certainly help here. But it would mean that argh64v8 also
> > > needs to define that, even though that is already RCsc.
> >
> > Maybe. It could also be that arm64 avoids the need somehow, for example
> > via their RCsc behavior. Their memory model is similar to PPC, but not
> > exactly the same.
> >
> > Will?
>
> So when I spoke to Will earlier today, we agreed that LDAXR+STXR is
> susceptible to the same problem. The STXR will allow loads to pass up
> over that store.
>
> On v8.1, which is using LDADDA, this is not an issue, for as the ACQUIRE
> is part of the LOAD, the Read-Modify-Write is indivisible as a whole,
> and therefore a subsequent load has nothing to pass over.

So one approach required for one level of hardware and another for the
next level. I can relate to that all too well... :-/

Thanx, Paul