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

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Message-ID: <20131210170404.GB23506@redhat.com>
Date:	Tue, 10 Dec 2013 18:04:04 +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 12/09, Paul E. McKenney wrote:
>
> This commit therefore adds a smp_mb__after_unlock_lock(), which may be
> placed after a LOCK primitive to restore the full-memory-barrier semantic.
> All definitions are currently no-ops, but will be upgraded for some
> architectures when queued locks arrive.

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.

Oleg.

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