linux-kernel - Re: NMI between switch_mm and switch

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Message-ID: <20090803082922.GB12498@elte.hu>
Date:	Mon, 3 Aug 2009 10:29:22 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Paul Mackerras <paulus@...ba.org>, linux-kernel@...r.kernel.org
Subject: Re: NMI between switch_mm and switch_to


* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> 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.

Yeah.

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

I think we should only disable user call-chains i think - the 
in-kernel call-chain is still reliable.

Also, i think we dont need preempt notifiers, we can use a simple 
check like this:

	if (current->mm &&
		cpu_isset(smp_processor_id(), &current->mm->cpu_vm_mask) {

		...
	}

In the user-call-chain code. We'd only touch the user memory image 
if that bit is set. cpu_vm_mask is maintained atomically and before 
we switch the MM, so it should be race-free.

	Ingo
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