Re: [GIT PULL rcu/next] rcu commits for 2.6.40

From: Ingo Molnar
Date: Mon May 16 2011 - 03:09:02 EST

* Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:

> > Would it have been possible to split it in two, one for the movement of the
> > notifiers, the other for the barrier changes?
> >
> > That way the bisection would have fingered the movement commit. Or so.
> In hindsight, that certainly would have been better.

This is the Linux kernel and we *can* turn back the clock!

> I was afraid of that...
> On the off-chance that moving the memory barriers was at fault, the following
> patch restores all of them that don't have in situ replacements. Grasping at
> straws, admittedly.

Well, the nice thing is that we really do not have to grasp at straws, and even
while we have no good ideas we can debug this *much* better.

Could you please do a simple test-tree that does has 3 commits:

first one reverts the offending commit
second one applies the barrier part of it
this one applies the need_resched part of it

( You can do even more finegrained steps, if you find harmless-looking bits of
it that can be applied separately! )

Note, the important thing is that the tree should be a 'null pull' - i.e. the
revert plus the patches applied will not change anything in core/rcu.

Obviously it would be nice if each step built fine - no need to boot test each
step as long as you are reasonably sure it will boot fine.

Then i could take my reproducer and come up with a very precise bisection
result for you, with just a couple of minutes time spent on testing. One of the
commits after the revert will trigger the hang/slowdown.

My prediction is that we will be much wiser after that! :-)


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