linux-kernel - Re: [PATCH v5 tip/core/locking 6/7] locking: Add an smp_mb__after_unlock

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Message-ID: <20131210173250.GA25934@redhat.com>
Date:	Tue, 10 Dec 2013 18:32:51 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu, Linux-Arch <linux-arch@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v5 tip/core/locking 6/7] locking: Add an
	smp_mb__after_unlock_lock() for UNLOCK+LOCK barrier

On 12/10, Paul E. McKenney wrote:
>
> On Tue, Dec 10, 2013 at 06:04:04PM +0100, Oleg Nesterov wrote:
> >
> > I am wondering, perhaps smp_mb__after_unlock() makes more sense?
> >
> > Note that it already has the potential user:
> >
> > 	--- x/kernel/sched/wait.c
> > 	+++ x/kernel/sched/wait.c
> > 	@@ -176,8 +176,9 @@ prepare_to_wait(wait_queue_head_t *q, wa
> > 		spin_lock_irqsave(&q->lock, flags);
> > 		if (list_empty(&wait->task_list))
> > 			__add_wait_queue(q, wait);
> > 	-	set_current_state(state);
> > 	+	__set_current_state(state);
> > 		spin_unlock_irqrestore(&q->lock, flags);
> > 	+	smp_mb__after_unlock();
> > 	 }
> > 	 EXPORT_SYMBOL(prepare_to_wait);
> >
> > 	@@ -190,8 +191,9 @@ prepare_to_wait_exclusive(wait_queue_hea
> > 		spin_lock_irqsave(&q->lock, flags);
> > 		if (list_empty(&wait->task_list))
> > 			__add_wait_queue_tail(q, wait);
> > 	-	set_current_state(state);
> > 	+	__set_current_state(state);
> > 		spin_unlock_irqrestore(&q->lock, flags);
> > 	+	smp_mb__after_unlock();
> > 	 }
> > 	 EXPORT_SYMBOL(prepare_to_wait_exclusive);
> >
> >
> > Assuming it can also be used "later", after another LOCK, like in
> > your example in 5/7.
>
> I am fine either way.  But there was an objection to tying this to the
> unlock because it costs more on many architectures than tying this to
> the lock.

OK, I see, thanks.

> But if you are saying "in addition to" rather than "instead of" that
> would be a different conversation.

Yes, please forget for now.

Oleg.

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