linux-kernel - Re: Linux 2.6.32-rc1

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Message-ID: <20090928225623.2dc5d2b2@mschwide.boeblingen.de.ibm.com>
Date:	Mon, 28 Sep 2009 22:56:23 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <johnstul@...ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: Linux 2.6.32-rc1

On Mon, 28 Sep 2009 20:41:41 +0200
Eric Dumazet <eric.dumazet@...il.com> wrote:

> I did a bisection and found commit def0a9b2573e00ab0b486cb5382625203ab4c4a6
> was the origin of the problem on my x86_32 machine.
> 
> def0a9b2573e00ab0b486cb5382625203ab4c4a6 is first bad commit
> commit def0a9b2573e00ab0b486cb5382625203ab4c4a6
> Author: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Date:   Fri Sep 18 20:14:01 2009 +0200
> 
>     sched_clock: Make it NMI safe
> 
>     Arjan complained about the suckyness of TSC on modern machines, and
>     asked if we could do something about that for PERF_SAMPLE_TIME.
> 
>     Make cpu_clock() NMI safe by removing the spinlock and using
>     cmpxchg. This also makes it smaller and more robust.
> 
>     Affects architectures that use HAVE_UNSTABLE_SCHED_CLOCK, i.e. IA64
>     and x86.
> 
>     Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
>     LKML-Reference: <new-submission>
>     Signed-off-by: Ingo Molnar <mingo@...e.hu>

Confirmed. The bisect run on my machine gave me the same bad commit.
The new logic in sched_clock_remove seems racy: the old code got the
locks for the sched_clock_data of the local and the remove cpu before
it changed any value. The new code tries to get to the same result with
a single cmpxchg. Bad things happen if two cpus try to update the clock
values crosswise, no?

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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