linux-kernel - Re: [PATCH] fix pgd

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Message-ID: <alpine.LFD.2.00.1102152020590.26192@localhost6.localdomain6>
Date:	Tue, 15 Feb 2011 20:26:51 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andrea Arcangeli <aarcange@...hat.com>
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, 15 Feb 2011, Andrea Arcangeli wrote:

> Hello,
> 
> Without this patch we can deadlock in the page_table_lock with NR_CPUS
> < 4 or THP on, with this patch we hopefully won't deadlock in the
> pgd_lock (if taken from irq). I can't see anything taking it from irq
> (maybe aio? to check I also tried the libaio testuite with no apparent
> VM_BUG_ON triggering), so unless somebody sees it, I think we should
> apply it. I've been running for a while with this patch applied
> without apparent problems. Other archs may follow suit if it's proven
> that there's nothing taking the pgd_lock from irq.
> 
> ===
> Subject: fix pgd_lock deadlock
> 
> From: Andrea Arcangeli <aarcange@...hat.com>
> 
> It's forbidden to take the page_table_lock with the irq disabled or if there's
> contention the IPIs (for tlb flushes) sent with the page_table_lock held will
> never run leading to a deadlock.

I really read this thing 5 times and still cannot make any sense of it.

You talk about page_table_lock and then fiddle with pgd_lock.

-ENOSENSE

	tglx

 
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