linux-kernel - Re: [COUNTERPATCH] mm: avoid overflowing preempt_count() in mmu_take_all

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Date:	Thu, 1 Apr 2010 17:36:45 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Avi Kivity <avi@...hat.com>, Thomas Gleixner <tglx@...utronix.de>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org,
	Kent Overstreet <kent.overstreet@...il.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [COUNTERPATCH] mm: avoid overflowing preempt_count() in
 mmu_take_all_locks()

On Thu, Apr 01, 2010 at 01:16:32PM +0200, Peter Zijlstra wrote:
> On Thu, 2010-04-01 at 14:13 +0300, Avi Kivity wrote:
> 
> > If someone is willing to audit all code paths to make sure these locks 
> > are always taken in schedulable context I agree that's a better fix.
> 
> They had better be, they're not irq-safe. Also that's what lockdep is
> for.

In my original patchset I included patches from Christoph to convert
those locks to mutexes, there was apparently no problem at all with
that. But frankly I think the only problem here is the warning. The
only compliant we ever had here is from developers, no users at
all. If this was a practical problem I think we should have heard
something by now with so many KVM users out there (and gru too).

The only single reason I'd go for mutexes would be to accommodate
XPMEM requirements once and for all, no other reason.
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