Re: [patch] x86, mm: avoid stale tlb entries by clearing prevmm_cpumask after switching mm

[Linus Torvalds]

On Wed, Feb 2, 2011 at 5:55 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Feb 2, 2011 at 12:07 PM, Suresh Siddha
> <suresh.b.siddha@xxxxxxxxx> 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.

Actually, for normal entries I could well imagine the code that wants
to do a flush before freeing the page caring.

So I think the _patch_ may be correct, but the changelog is definitely
not correct, and needs serious surgery to explain what the bug that
this fixes actually is.

                         Linus
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