linux-kernel - Re: [BUG] race of RCU vs NOHU

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Message-ID: <20090810142535.3e685109@skybase>
Date:	Mon, 10 Aug 2009 14:25:35 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Gerald Schaefer <gerald.schaefer@...ibm.com>,
	manfred@...orfullife.com
Subject: Re: [BUG] race of RCU vs NOHU

On Fri, 7 Aug 2009 07:29:57 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> On Fri, Aug 07, 2009 at 03:15:29PM +0200, Martin Schwidefsky wrote:
> > Hi Paul,
> > I analysed a dump of a hanging 2.6.30 system and found what I think is
> > a bug of RCU vs NOHZ. There are a number of patches ontop of that
> > kernel but they should be independent of the bug.
> > 
> > The systems has 4 cpus and uses classic RCU. cpus #0, #2 and #3 woke up
> > recently, cpu #1 has been sleeping for 5 minutes, but there is a pending
> > rcu batch. The timer wheel for cpu #1 is empty, it will continue to
> > sleep for NEXT_TIMER_MAX_DELTA ticks.
> 
> Congratulations, Martin!  You have exercised what to date has been a
> theoretical bug identified last year by Manfred Spraul.  The fix is to
> switch from CONFIG_RCU_CLASSIC to CONFIG_RCU_TREE, which was added in
> 2.6.29.
> 
> Of course, if you need to work with an old kernel version, you might
> still need a patch, perhaps for the various -stable versions.  If so,
> please let me know -- otherwise, I will focus forward on CONFIG_RCU_TREE
> rather than backwards on CONFIG_RCU_CLASSIC.

SLES11 is 2.6.27 and uses classic RCU. The not-so theoretical bug is
present there and I think it needs to be fixed :-/

-- 
blue skies,
   Martin.

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

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