From: Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [PATCH] fix pgd_lock deadlock
Date: Wed, 16 Feb 2011 11:15:21 +0100
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To: Peter Zijlstra <peterz@infradead.org>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>, "Xen-devel@lists.xensource.com" <Xen-devel@lists.xensource.com>, Ian Campbell <Ian.Campbell@citrix.com>, the arch/x86 maintainers <x86@kernel.org>, Hugh Dickins <hughd@google.com>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Jan Beulich <JBeulich@novell.com>, Andi Kleen <ak@suse.de>, Andrew Morton <akpm@linux-foundation.org>, Johannes Weiner <jweiner@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>, Larry Woodman <lwoodman@redhat.com>
List-Id: xen-devel@lists.xenproject.org

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.