linux-kernel - Re: PATCH? hrtimer_wakeup: fix a theoretical race wrt rt_mutex

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Date:	Mon, 06 Nov 2006 19:57:51 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Linus Torvalds <torvalds@...l.org>
Cc:	Oleg Nesterov <oleg@...sign.ru>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org
Subject: Re: PATCH? hrtimer_wakeup: fix a theoretical race wrt
	rt_mutex_slowlock()


> Yes. On x86 (and x86-64) you'll never see this, because writes are always 
> seen in order regardless, and in addition, the spin_lock is actually 
> totally serializing anyway. On most other architectures, the spin_lock 
> will serialize all the writes too, but it's not guaranteed, so in theory 
> you're right. I suspect no actual architecture will do this, but hey, 
> when talking memory ordering, safe is a lot better than sorry.

PowerPC doesn't serialize the writes on spin_lock, only on spin_unlock.

(That is, previous writes can "leak" into the lock, but writes done
before the unlock can't leak out of the spinlock).

Now, I've just glanced at the thread, so I don't know if that's relevant
to the problems you guys are talking about :-)

Ben.


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