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[Xen-devel] Re: [PATCH 08/13] xen/pvticketlock: disable interrupts while

To: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Subject: [Xen-devel] Re: [PATCH 08/13] xen/pvticketlock: disable interrupts while blocking
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Fri, 02 Sep 2011 12:29:37 -0700
Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>, Nick Piggin <npiggin@xxxxxxxxx>, KVM <kvm@xxxxxxxxxxxxxxx>, the arch/x86 maintainers <x86@xxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Andi Kleen <andi@xxxxxxxxxxxxxx>, Avi Kivity <avi@xxxxxxxxxx>, Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, Ingo Molnar <mingo@xxxxxxx>, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>, Xen Devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
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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@xxxxxxxxxx>
>>
>> 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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