linux-kernel - Re: [PATCH] fix pgd

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Date:	Tue, 15 Feb 2011 23:52:34 +0100
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	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

On Tue, Feb 15, 2011 at 09:26:35PM +0100, Thomas Gleixner wrote:
> Another thing. You check for in_interrupt(), but what makes sure that
> the code which takes pgd_lock is never taken with interrupts disabled
> except during early boot ?

It's perfectly fine to take pgd_lock with irq disabled, as long as you
don't pretend to take the page_table_lock too after that. So that's
not a concern.

I removed _irqsave from all pgd_lock, and I doubt there's any code
protected by pgd_lock that runs with irq disabled, but if there is,
it's still ok and it especially shouldn't have used _irqsave.

The only real issue here to sort out, is if pgd_lock is ever taken
from irq or not, and to me it looks like in_interrupt() should trigger
if it is ever taken from irq, so it won't go unnoticed for long if
this isn't ok.
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