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

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Message-Id: <1296698151.4418.464.camel@sbsiddha-MOBL3.sc.intel.com>
Date:	Wed, 02 Feb 2011 17:55:51 -0800
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Andi Kleen <andi@...stfloor.org>
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

On Wed, 2011-02-02 at 17:23 -0800, Andi Kleen wrote:
> 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.

I thought "asm volatile" is going to take care of that.

If not, then we have issues even today. no?

thanks,
suresh

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