Re: [PATCH -v2 4/6] locking, arch: Update spin_unlock_wait()

Peter Zijlstra
2016-05-27T09:07:19Z

On Thu, May 26, 2016 at 05:10:36PM -0400, Chris Metcalf wrote:
> On 5/26/2016 10:19 AM, Peter Zijlstra wrote:
> >--- a/arch/tile/lib/spinlock_32.c
> >+++ b/arch/tile/lib/spinlock_32.c
> >@@ -72,10 +72,14 @@ void arch_spin_unlock_wait(arch_spinlock
> > if (next == curr)
> > return;
> >+ smp_rmb();
> >+
> > /* Wait until the current locker has released the lock. */
> > do {
> > delay_backoff(iterations++);
> > } while (READ_ONCE(lock->current_ticket) == curr);
> >+
> >+ smp_acquire__after_ctrl_dep();
> > }
> > EXPORT_SYMBOL(arch_spin_unlock_wait);
> >--- a/arch/tile/lib/spinlock_64.c
> >+++ b/arch/tile/lib/spinlock_64.c
> >@@ -72,10 +72,14 @@ void arch_spin_unlock_wait(arch_spinlock
> > if (arch_spin_next(val) == curr)
> > return;
> >+ smp_rmb();
> >+
> > /* Wait until the current locker has released the lock. */
> > do {
> > delay_backoff(iterations++);
> > } while (arch_spin_current(READ_ONCE(lock->lock)) == curr);
> >+
> >+ smp_acquire__after_ctrl_dep();
> > }
> > EXPORT_SYMBOL(arch_spin_unlock_wait);
>
> The smp_rmb() are unnecessary for tile. We READ_ONCE next/curr from the
> lock and compare them, so we know the load(s) are complete. There's no
> microarchitectural speculation going on so that's that. Then we READ_ONCE
> the next load on the lock from within the wait loop, so our load/load
> ordering is guaranteed.

Does TILE never speculate reads? Because in that case the control
dependency already provides a full load->load,store barrier and you'd
want smp_acquire__after_ctrl_dep() to be a barrier() instead of
smp_rmb().