Re: [PATCH v3 5/6] pv-qspinlock: use cmpxchg_release in __pv_queued_spin_unlock

[xinhui]

On 2016å05æ27æ 00:57, Peter Zijlstra wrote:
> 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@xxxxxxxxx>
> > > Signed-off-by: Pan Xinhui <xinhui.pan@xxxxxxxxxxxxxxxxxx>
> > > ---
> > >   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.
Never mind.  thanks for review :)

> Still Changelog needs improvement.
Will do that.

thanks
xinhui