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

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Message-ID: <Pine.LNX.4.58.0611060732020.14553@gandalf.stny.rr.com>
Date:	Mon, 6 Nov 2006 07:35:03 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
cc:	Linus Torvalds <torvalds@...l.org>,
	Oleg Nesterov <oleg@...sign.ru>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org
Subject: Re: PATCH? hrtimer_wakeup: fix a theoretical race wrt rt_mutex_slowlock()

On Mon, 6 Nov 2006, Benjamin Herrenschmidt wrote:

>
> > 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 :-)
>

It is relevant.  In powerpc, can one write happen before another write?


  x = 1;
  barrier();  (only compiler barrier)
  b = 2;


And have CPU 2 see b=2 before seeing x=1?

If so, then I guess this is indeed a bug on powerpc.

-- Steve

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