[PATCH v3 5/6] pv-qspinlock: use cmpxchg_release in __pv_queued_spin_unlock
peterz at infradead.org
Fri May 27 02:57:12 AEST 2016
On Thu, May 26, 2016 at 06:47:29PM +0200, Peter Zijlstra wrote:
> On Wed, May 25, 2016 at 04:18:08PM +0800, Pan Xinhui wrote:
> > cmpxchg_release is light-wight than cmpxchg, we can gain a better
> > performace then. On some arch like ppc, barrier impact the performace
> > too much.
> > Suggested-by: Boqun Feng <boqun.feng at gmail.com>
> > Signed-off-by: Pan Xinhui <xinhui.pan at linux.vnet.ibm.com>
> > ---
> > kernel/locking/qspinlock_paravirt.h | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> > diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
> > index a5b1248..2bbffe4 100644
> > --- a/kernel/locking/qspinlock_paravirt.h
> > +++ b/kernel/locking/qspinlock_paravirt.h
> > @@ -614,7 +614,7 @@ __visible void __pv_queued_spin_unlock(struct qspinlock *lock)
> > * unhash. Otherwise it would be possible to have multiple @lock
> > * entries, which would be BAD.
> > */
> > - locked = cmpxchg(&l->locked, _Q_LOCKED_VAL, 0);
> > + locked = cmpxchg_release(&l->locked, _Q_LOCKED_VAL, 0);
> > if (likely(locked == _Q_LOCKED_VAL))
> > return;
> This patch fails to explain _why_ it can be relaxed.
> And seeing how this cmpxchg() can actually unlock the lock, I don't see
> how this can possibly be correct. Maybe cmpxchg_release(), but relaxed
> seems very wrong.
Clearly I need to stop working for the day, I cannea read. You're doing
release, not relaxed.
Still Changelog needs improvement.
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