Re: [PATCH] Fix locking bug in vcpu_migrate

[John Weekes <lists.xen@nuclearfallout.net>]

> Nice that empirical evidence supports the patch, however, I'm being dense
> and don't understand why order of lock release matters.

I was taught long ago to release them in order, but as I think about it, 
I agree with you that there doesn't seem to be a scenario when the 
release would matter.

It's odd that it seemed to lead to such a big difference for me, then.  
I'll do some further tests -- maybe I changed something else to cause 
the behavior, or the problem is more random than I thought and just 
hasn't occurred for me yet in all the new tests.

> perhaps you've merely perturbed a fragile pos code that's
> broken in some other way.

That's entirely possible.

> Also the last hunk of your patch is broken -- in the final else clause you
> call spin_unlock_irqrestore on the wrong lock. This is very definitely a
> bug, as irqs should never be enabled while any schedule_lock is held.

Definitely. I had fixed that here but sent an old version of the patch 
-- a boneheaded mistake.

-John