linux-kernel - Re: futex code and barriers

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Message-ID: <Pine.LNX.4.64.0804291442050.22014@twin.jikos.cz>
Date:	Tue, 29 Apr 2008 14:43:12 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>,
	Paul E McKenney <paulmck@...ux.vnet.ibm.com>,
	Oleg Nesterov <oleg@...sign.ru>, Nick Piggin <npiggin@...e.de>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: futex code and barriers

On Tue, 29 Apr 2008, Peter Zijlstra wrote:

> While looking through the futex code I stumbled upon the following bit:
> kernel/futex.c:
> 	/* add_wait_queue is the barrier after __set_current_state. */
> 	__set_current_state(TASK_INTERRUPTIBLE);
> 	add_wait_queue(&q.waiters, &wait);
> However,
> void add_wait_queue(wait_queue_head_t *q, wait_queue_t *wait)
> {
> 	unsigned long flags;
> 
> 	wait->flags &= ~WQ_FLAG_EXCLUSIVE;
> 	spin_lock_irqsave(&q->lock, flags);
> 	__add_wait_queue(q, wait);
> 	spin_unlock_irqrestore(&q->lock, flags);
> }
[ ... ]
> Non of which implies a full barrier.

Well I am probably missing the point, but what about the lock and unlock 
of the spinlock?

-- 
Jiri Kosina
SUSE Labs

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