linux-kernel - Re: [tip:perfcounters/core] x86: Add NMI types for kmap

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Date:	Mon, 15 Jun 2009 17:02:46 +0100 (BST)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
	paulus@...ba.org, acme@...hat.com, efault@....de, npiggin@...e.de,
	tglx@...utronix.de, linux-tip-commits@...r.kernel.org,
	mathieu.desnoyers@...ymtl.ca, torvalds@...ux-foundation.org,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [tip:perfcounters/core] x86: Add NMI types for kmap_atomic

On Mon, 15 Jun 2009, Ingo Molnar wrote:
> 
> Note that Oprofile uses the same method of __copy_user_inatomic() in 
> arch/x86/oprofile/backtrace.c, but i believe that code is broken - i 
> doubt the call-chain support for user-space stacks ever worked in 
> oprofile - with perfcounters i can make this method crash under 
> load. (we re-enter the NMI which due to IST executes over the exact 
> same, still pending NMI frame. Kaboom.)
> 
> I saw you being involved with the Oprofile code 3 years ago:
> 
> | commit c34d1b4d165c67b966bca4aba026443d7ff161eb
> | Author: Hugh Dickins <hugh@...itas.com>
> | Date:   Sat Oct 29 18:16:32 2005 -0700
> |
> |    [PATCH] mm: kill check_user_page_readable
> 
> That method of __copy_user_inatomic(), while elegant, is subtly 
> wrong in an NMI context. We really must avoid taking faults there.

Yes, I'm afraid that subtlety escaped me - thanks for explaining.

Hugh
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