linux-kernel - Re: [PATCH v3 05/11] x86/mm: Track the TLB's tlb

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Date:   Thu, 22 Jun 2017 07:48:21 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Nadav Amit <nadav.amit@...il.com>,
        Rik van Riel <riel@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 05/11] x86/mm: Track the TLB's tlb_gen and update the
 flushing algorithm

On Thu, Jun 22, 2017 at 12:24 AM, Borislav Petkov <bp@...en8.de> wrote:
> On Wed, Jun 21, 2017 at 07:46:05PM -0700, Andy Lutomirski wrote:
>> > I'm certainly still missing something here:
>> >
>> > We have f->new_tlb_gen and mm_tlb_gen to control the flushing, i.e., we
>> > do once
>> >
>> >         bump_mm_tlb_gen(mm);
>> >
>> > and once
>> >
>> >         info.new_tlb_gen = bump_mm_tlb_gen(mm);
>> >
>> > and in both cases, the bumping is done on mm->context.tlb_gen.
>> >
>> > So why isn't that enough to do the flushing and we have to consult
>> > info.new_tlb_gen too?
>>
>> The issue is a possible race.  Suppose we start at tlb_gen == 1 and
>> then two concurrent flushes happen.  The first flush is a full flush
>> and sets tlb_gen to 2.  The second is a partial flush and sets tlb_gen
>> to 3.  If the second flush gets propagated to a given CPU first and it
>
> Maybe I'm still missing something, which is likely...
>
> but if the second flush gets propagated to the CPU first, the CPU will
> have local tlb_gen 1 and thus enforce a full flush anyway because we
> will go 1 -> 3 on that particular CPU. Or?
>

Yes, exactly.  Which means I'm probably just misunderstanding your
original question.  Can you re-ask it?

--Andy