On Thu, Mar 26, 2015 at 02:41:54PM +0000, Will Deacon wrote:
> > +++ b/lib/lockref.c
> > @@ -18,7 +18,8 @@
> > #define CMPXCHG_LOOP(CODE, SUCCESS) do {
> > \
> > struct lockref old;
> > \
> > BUILD_BUG_ON(sizeof(old) != 8);
> > \
> > - old.lock_count = READ_ONCE(lockref->lock_count);
> > \
> > + barrier();
> > \
> > + old.lock_count = lockref->lock_count;
> > \
> > while (likely(arch_spin_value_unlocked(old.lock.rlock.raw_lock))) {
> > \
> > struct lockref new = old, prev = old;
> > \
> > CODE
> > \
>
> Is ACCESS_ONCE actually going away?
I've been arguing for that yes, having two APIs for the 'same' thing is
confusing at best, and as the comment near the READ_ONCE() thing
explains, ACCESS_ONCE() has serious, silent, issues.
> It has its problems, but I think it's
> what we want here and reads better than magic barrier() imo.
Yeah, but its also misleading because we rely on silent fail. Part of
the ACCESS_ONCE() semantics is that it should avoid split loads, and
we're here actually relying on emitting just that.
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