linux-kernel - Re: [PATCH] fix pgd

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Date:	Wed, 16 Feb 2011 11:28:01 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	"Xen-devel@...ts.xensource.com" <Xen-devel@...ts.xensource.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	Jan Beulich <JBeulich@...ell.com>,
	Larry Woodman <lwoodman@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <ak@...e.de>, Johannes Weiner <jweiner@...hat.com>,
	Hugh Dickins <hughd@...gle.com>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH] fix pgd_lock deadlock


* Andrea Arcangeli <aarcange@...hat.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
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