linux-kernel - Re: Mysterious CFQ crash and RCU

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Date:	Tue, 24 May 2011 07:35:27 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jens Axboe <jaxboe@...ionio.com>
Cc:	Paul Bolle <pebolle@...cali.nl>, Vivek Goyal <vgoyal@...hat.com>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: Mysterious CFQ crash and RCU

On Tue, May 24, 2011 at 11:41:10AM +0200, Jens Axboe wrote:
> On 2011-05-24 00:20, Paul Bolle wrote:
> > On Mon, 2011-05-23 at 08:38 -0700, Paul E. McKenney wrote:
> >> Running under CONFIG_PREEMPT=y (along with CONFIG_TREE_PREEMPT_RCU=y)
> >> could be very helpful in and of itself.  CONFIG_DEBUG_OBJECTS_RCU_HEAD=y
> >> can also be helpful.  In post-2.6.39 mainline, it should be possible
> >> to set CONFIG_DEBUG_OBJECTS_RCU_HEAD=y without CONFIG_PREEMPT=y, but
> >> again, CONFIG_PREEMPT=y can help find problems.
> > 
> > 0) The first thing I tried (from your suggestions) was
> > CONFIG_DEBUG_OBJECTS_RCU_HEAD=y. Given its dependencies (and, well, the
> > build system I used) I ended up with:
> > 
> >     $ grep -e PREEMPT -e RCU /boot/config-2.6.39-0.local3.fc16.i686 |
> >     grep -v "^#"
> >     CONFIG_TREE_PREEMPT_RCU=y
> >     CONFIG_PREEMPT_RCU=y
> >     CONFIG_RCU_FANOUT=32
> >     CONFIG_PREEMPT_NOTIFIERS=y
> >     CONFIG_PREEMPT=y
> >     CONFIG_DEBUG_OBJECTS_RCU_HEAD=y
> >     CONFIG_DEBUG_PREEMPT=y
> >     CONFIG_PROVE_RCU=y
> >     CONFIG_SPARSE_RCU_POINTER=y
> > 
> > It looks like I am unable to trigger the issue we're talking about here
> > when using that config.
> > 
> > 1) For reference, the config of a kernel that does trigger it had:
> > 
> >     $ grep -e PREEMPT -e RCU /boot/config-2.6.39-0.local2.fc16.i686 |
> >     grep -v "^#"
> >     CONFIG_TREE_RCU=y
> >     CONFIG_RCU_FANOUT=32
> >     CONFIG_RCU_FAST_NO_HZ=y
> >     CONFIG_PREEMPT_NOTIFIERS=y
> >     CONFIG_PREEMPT_VOLUNTARY=y
> >     CONFIG_PROVE_RCU=y
> >     CONFIG_SPARSE_RCU_POINTER=y
> > 
> >>> Again CONFIG_TREE_PREEMPT_RCU is available only if PREEMPT=y. So should
> >>> we enable preemtion and CONFIG_TREE_PREEMPT_RCU=y and try to reproduce
> >>> the issue?
> >>
> >> Please!
> > 
> > 2) It appears I can't reproduce with those options enabled (see above).
> > 
> >> Polling is fine.  Please see attached for a script to poll at 15-second
> >> intervals.  Please also feel free to adjust, just tell me what you
> >> adjusted.
> > 
> > And should I now try to run that script on a config that triggers this
> > issue (such as the config under 1) above)?
> 
> Paul, can we see a dmesg from your running system? Perhaps there's some
> dependency on a particular driver or device that makes this easier to
> reproduce.

Here you go, please see attached.

I should have some additional diagnostics later today Pacific time.

							Thanx, Paul

View attachment "console.log" of type "text/plain" (100662 bytes)