Subject: Re: NMI between switch_mm and switch_to
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@elte.hu>, linux-kernel@vger.kernel.org
In-Reply-To: <19054.33655.297932.261580@cargo.ozlabs.ibm.com>
References: <19054.33655.297932.261580@cargo.ozlabs.ibm.com>
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Date: Tue, 28 Jul 2009 09:51:12 +0200
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On Tue, 2009-07-28 at 14:49 +1000, Paul Mackerras wrote:
> Ben H. suggested there might be a problem if we get a PMU interrupt
> and try to do a stack trace of userspace in the interval between when
> we call switch_mm() from sched.c:context_switch() and when we call
> switch_to().  If we get an NMI in that interval and do a stack trace
> of userspace, we'll see the registers of the old task but when we peek
> at user addresses we'll see the memory image for the new task, so the
> stack trace we get will be completely bogus.
> 
> Is this in fact also a problem on x86, or is there some subtle reason
> why it can't happen there?

I can't spot one, maybe Ingo can when he's back :-)

So I think this is very good spotting from Ben.

We could use preempt notifiers (or put in our own hooks) to disable
callchains during the context switch I suppose.

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