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

From: Paul E. McKenney
Date: Mon May 16 2011 - 03:48:29 EST

On Mon, May 16, 2011 at 09:08:08AM +0200, Ingo Molnar wrote:
> * 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!

Yay for source-code control systems in general and git in particular! ;-)

> > 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! :-)

I will put this together!

In the meantime, would you be willing to try out the patch at This patch helped out Yinghai
in several configurations.

Thanx, Paul
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