Re: frequent lockups in 3.18rc4

From: Paul E. McKenney
Date: Tue Dec 02 2014 - 13:42:12 EST

On Tue, Dec 02, 2014 at 03:14:08PM -0200, Dâniel Fraga wrote:
> On Tue, 2 Dec 2014 09:04:07 -0800
> "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> > Is it harder to reproduce with CONFIG_PREEMPT=y and CONFIG_TREE_PREEMPT_RCU=n?
> Yes, it's much harder! :)
> > If it is a -lot- harder to reproduce, it might be worth bisecting among
> > the RCU read-side critical sections. If making a few of them be
> > non-preemptible greatly reduces the probability of the bug occuring,
> > that might provide a clue about root cause.
> >
> > On the other hand, if it is just a little harder to reproduce, this
> > RCU read-side bisection would likely be an exercise in futility.
> Ok, I want to bisect it. Since it could be painful to bisect,
> could you suggest 2 commits between 3.16.0 and 3.17.0 so we can narrow
> the bisect? I could just bisect between 3.16.0 and 3.17.0 but it would
> take many days :).
> Ps: if you prefer I bisect between 3.16.0 and 3.17.0, no
> problem, but you'll have to be patient ;).

I was actually suggesting something a bit different. Instead of bisecting
by release, bisect by code. The procedure is as follows:

1. I figure out some reliable way of making RCU allow preemption to
be disabled for some RCU read-side critical sections, but not for
others. I send you the patch, which has rcu_read_lock_test()
as well as rcu_read_lock().

2. You build a kernel without my Kconfig hack, with my patch from
#1 above, and build a kernel with CONFIG_PREEMPT=y (which of
course implies CONFIG_TREE_PREEMPT_RCU=y, given that you are
building without my Kconfig hack).

3. You make a list of all the rcu_read_lock() uses in the kernel
(or ask me to provide it). You change the rcu_read_lock()
calls in the first half of this list to rcu_read_lock_test().

If the kernel locks up as easily with this change as it did
change half of the remaining rcu_read_lock() calls to
rcu_read_lock_test(). If the kernel is much more resistant
to lockup, change half of the rcu_read_lock_test() calls
back to rcu_read_lock().

4. It is quite possible that several of the RCU read-side critical
sections contribute to the unreliability, in which case the
bisection will get a bit more complicated.

Other thoughts on how to attack this?

Thanx, Paul

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