linux-kernel - Re: [PATCH 08/13] xen/pvticketlock: disable interrupts while blocking

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Message-ID: <4E612EA1.20007@goop.org>
Date:	Fri, 02 Sep 2011 12:29:37 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...nel.dk>, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	KVM <kvm@...r.kernel.org>, Andi Kleen <andi@...stfloor.org>,
	Xen Devel <xen-devel@...ts.xensource.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [PATCH 08/13] xen/pvticketlock: disable interrupts while blocking

On 09/02/2011 07:47 AM, Peter Zijlstra wrote:
> On Thu, 2011-09-01 at 17:55 -0700, Jeremy Fitzhardinge wrote:
>> From: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
>>
>> We need to make sure interrupts are disabled while we're relying on the
>> contents of the per-cpu lock_waiting values, otherwise an interrupt
>> handler could come in, try to take some other lock, block, and overwrite
>> our values.
> Would this make it illegal to take a spinlock from NMI context?

That would be problematic.  But a Xen domain wouldn't be getting NMIs -
at least not standard x86 ones - so that's moot.

> I know that its generally considered bad form, but there's at least one
> spinlock that's only taken from NMI context and thus hasn't got any
> deadlock potential.

Which one?

    J
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