linux-kernel - Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier

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Date:	Wed, 15 Jun 2016 16:04:46 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	Waiman Long <Waiman.Long@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, linux-alpha@...r.kernel.org,
	linux-ia64@...r.kernel.org, linux-s390@...r.kernel.org,
	linux-arch@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
	Jason Low <jason.low2@...com>,
	Dave Chinner <david@...morbit.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper
 acquire/release barrier

Hi Waiman,

On Tue, Jun 14, 2016 at 06:48:04PM -0400, Waiman Long wrote:
> The osq_lock() and osq_unlock() function may not provide the necessary
> acquire and release barrier in some cases. This patch makes sure
> that the proper barriers are provided when osq_lock() is successful
> or when osq_unlock() is called.
> 
> Signed-off-by: Waiman Long <Waiman.Long@....com>
> ---
>  kernel/locking/osq_lock.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index 05a3785..7dd4ee5 100644
> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -115,7 +115,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
>  	 * cmpxchg in an attempt to undo our queueing.
>  	 */
>  
> -	while (!READ_ONCE(node->locked)) {
> +	while (!smp_load_acquire(&node->locked)) {
>  		/*
>  		 * If we need to reschedule bail... so we can block.
>  		 */
> @@ -198,7 +198,7 @@ void osq_unlock(struct optimistic_spin_queue *lock)
>  	 * Second most likely case.
>  	 */
>  	node = this_cpu_ptr(&osq_node);
> -	next = xchg(&node->next, NULL);
> +	next = xchg_release(&node->next, NULL);
>  	if (next) {
>  		WRITE_ONCE(next->locked, 1);

So we still use WRITE_ONCE() rather than smp_store_release() here?

Though, IIUC, This is fine for all the archs but ARM64, because there
will always be a xchg_release()/xchg() before the WRITE_ONCE(), which
carries a necessary barrier to upgrade WRITE_ONCE() to a RELEASE.

Not sure whether it's a problem on ARM64, but I think we certainly need
to add some comments here, if we count on this trick.

Am I missing something or misunderstanding you here?

Regards,
Boqun

>  		return;
> -- 
> 1.7.1
> 

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