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

From: Peter Zijlstra <[email-masked]>
Date: Thu, 26 May 2016 18:47:29 +0200

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 <[email-masked]>
> Signed-off-by: Pan Xinhui <[email-masked]>
> ---
>  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.