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

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Date:	Wed, 2 Feb 2011 17:55:52 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.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>,
	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, Feb 2, 2011 at 12:07 PM, Suresh Siddha
<suresh.b.siddha@...el.com> wrote:
> 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.

Why?

This looks pointless. Explain why this matters. Global entries are
never per-mm, so any global entries can never care about the
mm_cpumask.

And for any normal entries it doesn't matter if the IPI gets lost,
since the TLB will be flushed (immediately afterwards) by the cr3
write.

So no, I will not accept this patch, and it shouldn't be marked for
stable. Unless you can show exactly what is the point of it, and
exactly what the bug is.

                       Linus
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