linux-kernel - Re: preempt bug in set_pmd

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Date:	Wed, 5 Mar 2008 17:38:28 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
cc:	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Andi Kleen <ak@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: preempt bug in set_pmd_pfn?

On Wed, 5 Mar 2008, Jeremy Fitzhardinge wrote:
> 
> If a thread goes from processor A -> B -> A, where A is first preempted
> between a pagetable update and a tlb flush, then the second time the thread
> runs on A may run with a stale tlb (if in the meantime A has either been idle
> or only running kernel threads).

Right, thanks, because __flush_tlb_one opts out of the full active_mm
checking which goes on for userspace mms (which covers, for example,
the case of preemption before dup_mmap's flush_tlb_mm).

But is there actually a case where there's a problem?  So far as I can see,
set_pmd_pfn is there solely for discontig_32's __init remap_numa_kva; and
set_pte_pfn is there solely for set_fixmap, which operates on a per-cpu
area of pagetable, which would already be in bigger trouble if preemption
to another cpu were possible.

Hugh
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