linux-kernel - Re: [patch] x86, mm: avoid stale tlb entries by clearing prev mm

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Date:	Wed, 02 Feb 2011 17:23:53 -0800
From:	Andi Kleen <andi@...stfloor.org>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Mallick\, Asit K" <asit.k.mallick@...el.com>
Subject: Re: [patch] x86, mm: avoid stale tlb entries by clearing prev mm_cpumask after switching mm

Suresh Siddha <suresh.b.siddha@...el.com> writes:

> For the prev mm that is handing over the cpu to another mm, clear the cpu
> from the mm_cpumask(prev) after the cr3 is changed.
>
> Otherwise, clearing the mm_cpumask early will avoid the flush tlb IPI's while
> the cr3 and TLB's are still pointing to the prev mm. And this window can lead
> to the stale (global) TLB entries.
>
> Marking it for -stable, though we haven't seen any reported failure that
> can be attributed to this.

Would it be safer to add a memory barrier between the load_cr3 and the
cpumask_clear_cpu()? As far as I can see cpumask_clear_cpu doesn't
imply a general one and load_cr3 doesn't either. There's this
__force_order hack in system.h, but I don't think it will enforce
order here.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only
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