linux-kernel - Re: [PATCH] bug in futex unqueue

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Date:	Mon, 31 Jul 2006 13:49:31 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Christian Borntraeger <borntrae@...ibm.com>
Cc:	linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...esys.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Andrew Morton <akpm@...l.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] bug in futex unqueue_me


* Christian Borntraeger <borntrae@...ibm.com> wrote:

> On Sunday 30 July 2006 08:38, Ingo Molnar wrote:
> > interesting, how is this possible? We do a spin_lock(lock_ptr), and
> > taking a spinlock is an implicit barrier(). So gcc must not delay
> > evaluating lock_ptr to inside the critical section. And as far as i can
> > see the s390 spinlock implementation goes through an 'asm volatile'
> > piece of code, which is a barrier already. So how could this have
> > happened?
> 
> spin_lock is a barrier, but isnt the barrierness too late here? The 
> compiler reloads the value of lock_ptr after the "if(lock_ptr)" and 
> *before* calling spin_lock(lock_ptr):

ah, indeed. So your patch is a real fix. Thanks,

Acked-by: Ingo Molnar <mingo@...e.hu>

	Ingo
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