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

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Date:   Thu, 22 Jun 2017 09:24:49 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     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 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?

-- 
Regards/Gruss,
    Boris.

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