linux-kernel - Re: [PATCH v3 5/6] pv-qspinlock: use cmpxchg_release in __pv_queued_spin

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Message-ID: <20160526164729.GL3206@twins.programming.kicks-ass.net>
Date:	Thu, 26 May 2016 18:47:29 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	virtualization@...ts.linux-foundation.org,
	benh@...nel.crashing.org, paulus@...ba.org, mpe@...erman.id.au,
	mingo@...hat.com, paulmck@...ux.vnet.ibm.com, jeremy@...p.org,
	chrisw@...s-sol.org, akataria@...are.com, rusty@...tcorp.com.au
Subject: Re: [PATCH v3 5/6] pv-qspinlock: use cmpxchg_release in
 __pv_queued_spin_unlock

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@...il.com>
> Signed-off-by: Pan Xinhui <xinhui.pan@...ux.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.