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

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Message-ID: <20090810161007.GB1505@ucw.cz>
Date:	Mon, 10 Aug 2009 18:10:07 +0200
From:	Pavel Machek <pavel@....cz>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:	paulmck@...ux.vnet.ibm.com, 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

Hi!

> > > 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 :-/

Plus... if config_rcu_classic is known buggy  (and you are not willing
to fix it),  it should be disabled (or made depend on !CONFIG_NOHZ, or
maybe made depend on CONFIG_BROKEN).

Ugh. rcu_classic  does not seem to have help text, and I'm running
								known
								bad
								code
								here
								:-(.


								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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