Re: [PATCH] fix pgd_lock deadlock

Ingo Molnar
2011-02-16T10:28:56Z

* Andrea Arcangeli <aarcange@redhat.com> wrote:

> On Wed, Feb 16, 2011 at 10:58:14AM +0100, Peter Zijlstra wrote:
> > On Wed, 2011-02-16 at 00:17 +0100, Andrea Arcangeli wrote:
> > > On Wed, Feb 16, 2011 at 12:03:30AM +0100, Thomas Gleixner wrote:
> > > > I assume you run it with a lockdep enabled kernel as well, right ?
> > >
> > > Yes, I always run with lockdep and prove locking enabled on my test
> > > box, not sure how it's meant to trigger more bugs in this case, the
> > > debug check that should be relevant for this is DEBUG_VM and that is
> > > enabled too of course. I didn't try DEBUG_PAGEALLOC yet.
> >
> > I think what Thomas tried to tell you is that your
> > VM_BUG_ON(in_interrupt()) is fully redundant if you have lockdep
> > enabled.
> >
> > Lockdep will warn you if a !irqsave lock is taken from IRQ context,
> > since that is a clear inversion problem.
>
> Ah I get it now, but I prefer to have it on an all my builds, and
> I don't keep lockdep on for all builds (but I keep DEBUG_VM on). It's
> still only debug code that no production system will ever deal with,
> so it should be good to exercise it in more than on debug .config
> considering it's very low overhead (pgd_lock is never taken in fast
> paths) so it's suitable for a VM_BUG_ON.

The point both Thomas and Peter tried to point out to you, that adding 7 instances
of redundant debug checks:

+ VM_BUG_ON(in_interrupt());
+ VM_BUG_ON(in_interrupt());
+ VM_BUG_ON(in_interrupt());
+ VM_BUG_ON(in_interrupt());
+ VM_BUG_ON(in_interrupt());
+ VM_BUG_ON(in_interrupt());
+ VM_BUG_ON(in_interrupt());

to arch/x86/ is not acceptable, for the reasons stated. Please remove it from the
patch.

Thanks,

Ingo