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SubjectRe: Mysterious CFQ crash and RCU
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Paul,

On Sat, 2011-05-21 at 14:00 -0700, Paul E. McKenney wrote:
> On Thu, May 19, 2011 at 06:24:04PM -0400, Vivek Goyal wrote:
> It does look like a tough one!

Thank you!

> > Is it possible? We have looked at the code many a times and we think
> > that rcu locking around it is fine. Is it possible that a call_rcu()
> > can fire before rcu grace period is over.
>
> If it does, that would be a bug in RCU.
>
> > I had put a debug patch in CFQ (details are in bugzilla) and I can
> > see that after decoupling the object from the hash list, it got
> > freed while we were still under rcu_read_lock().
> >
> > Is there any known issue or is there any quick tip on how can I
> > go about debugging it further from rcu point of view.
>
> First for uses of RCU:
>
> o One thing to try would be CONFIG_PROVE_RCU, which could help
> find missing rcu_read_lock()s and similar. Some years back, it
> used to be the case that spin_lock() implied rcu_read_lock(),
> but it no longer does. There might still be some cases where
> spin_lock() needs to have an rcu_read_lock() added.
>
> o There are a few entries in the bugzilla mentioning that elements
> are being removed more often than expected. There is a config
> option CONFIG_DEBUG_OBJECTS_RCU_HEAD that complains if the same
> object is passed to call_rcu() before the grace period ends for
> the first round.
>
> o Try switching between CONFIG_TREE_RCU and CONFIG_TREE_PREEMPT_RCU.
> These two settings are each sensitive to different forms of abuse.
> For example, if you have CONFIG_PREEMPT=n and CONFIG_TREE_RCU=y,
> illegally placing a synchronize_rcu() -- or anything else that
> blocks -- in an RCU read-side critical section will silently
> partition that RCU read-side critical section. In contrast,
> CONFIG_TREE_PREEMPT_RCU=y will complain about this.
>
> Second, for RCU itself, CONFIG_RCU_TRACE enables counter-based tracing
> in RCU. Sampling each of the files in the debugfs directory "rcu"
> before and after the badness (if possible) could help me see if anything
> untoward is happening.

Before we go down that route, I'd like to note that I seem to be unable
to reproduce this Oops under v2.6.39 (either using the first v2.6.39 rpm
for i686 shipped for Fedora Rawhide, or two versions of that rpm I built
locally).

Is anyone able to spot one or more commits in v2.6.39-rc7..v2.6.39 that
might have fixed this Oops? Or did my chance of hitting this Oops,
somehow, just got a lot smaller in v.2.6.39?

Please note that I have tried to reproduce this Oops very often, using
quite a number of kernels, so there's a non-zero chance I tricked myself
in seeing a pattern where there actually is none.


Paul Bolle



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