Re: [Xen-devel] [PATCH 05/11] qspinlock: Optimize for smaller NR_CPUS

[Paolo Bonzini <pbonzini@xxxxxxxxxx>]

Il 07/07/2014 16:35, Peter Zijlstra ha scritto:
> On Wed, Jun 18, 2014 at 01:39:52PM +0200, Paolo Bonzini wrote:
> > Il 15/06/2014 14:47, Peter Zijlstra ha scritto:
> > > -       for (;;) {
> > > -               new = (val & ~_Q_PENDING_MASK) | _Q_LOCKED_VAL;
> > > -
> > > -               old = atomic_cmpxchg(&lock->val, val, new);
> > > -               if (old == val)
> > > -                       break;
> > > -
> > > -               val = old;
> > > -       }
> > > +       clear_pending_set_locked(lock, val);
> > >         return;
> >
> > Might as well add clear_pending_set_locked already in patch 3.
> Patch 4, if anywhere.
This code is added in patch 3:

+       /*
+        * take ownership and clear the pending bit.
+        *
+        * *,1,0 -> *,0,1
+        */
+       for (;;) {
+               new = (val & ~_Q_PENDING_MASK) | _Q_LOCKED_VAL;
+
+               old = atomic_cmpxchg(&lock->val, val, new);
+               if (old == val)
+                       break;
+
+               val = old;
+       }
+       return;

Unlike the change in patch 4, clear_pending_set_locked doesn't change 
how qspinlock moves from a state to the next.
Paolo

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