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Date: Tue, 11 Aug 2009 14:23:52 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Pavel Machek <pavel@ucw.cz>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>, linux-kernel@vger.kernel.org,
       Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>,
       Gerald Schaefer <gerald.schaefer@de.ibm.com>, manfred@colorfullife.com
Subject: Re: [BUG] race of RCU vs NOHU
Message-ID: <20090811212352.GF6739@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
References: <20090807151529.2806b8b4@skybase> <20090807142957.GB6700@linux.vnet.ibm.com> <20090810142535.3e685109@skybase> <20090810161007.GB1505@ucw.cz>
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On Mon, Aug 10, 2009 at 06:10:07PM +0200, Pavel Machek wrote:
> 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).

Already done in -tip.  ;-)

	paulmck@paulmck-laptop:/home/git/linux-2.6-tip$ ls kernel/rcuclassic.c
	ls: kernel/rcuclassic.c: No such file or directory

							Thanx, Paul

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