Date: Tue, 15 Feb 2011 23:52:34 +0100
From: Andrea Arcangeli <aarcange@redhat.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>, "H. Peter Anvin" <hpa@zytor.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        "Xen-devel@lists.xensource.com" <Xen-devel@lists.xensource.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Ian Campbell <Ian.Campbell@citrix.com>,
        Jan Beulich <JBeulich@novell.com>, Larry Woodman <lwoodman@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>, Andi Kleen <ak@suse.de>,
        Johannes Weiner <jweiner@redhat.com>, Hugh Dickins <hughd@google.com>,
        Rik van Riel <riel@redhat.com>
Subject: Re: [PATCH] fix pgd_lock deadlock
Message-ID: <20110215225234.GP5935@random.random>
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 <20110215190710.GL5935@random.random>
 <alpine.LFD.2.00.1102152020590.26192@localhost6.localdomain6>
 <20110215195450.GO5935@random.random>
 <alpine.LFD.2.00.1102152102530.26192@localhost6.localdomain6>
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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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