linux-kernel - Re: [PATCH v6] x86/mm: Improve TLB flush documentation

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Message-ID: <20170726140341.rp5tckxv2cpk4uya@hirez.programming.kicks-ass.net>
Date:   Wed, 26 Jul 2017 16:03:41 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...en8.de>,
        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>
Subject: Re: [PATCH v6] x86/mm: Improve TLB flush documentation

On Wed, Jul 26, 2017 at 06:52:06AM -0700, Andy Lutomirski wrote:
> On Tue, Jul 25, 2017 at 7:44 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Tue, Jul 25, 2017 at 07:10:44AM -0700, Andy Lutomirski wrote:
> >> Improve comments as requested by PeterZ and also add some
> >> documentation at the top of the file.
> >>
> >> This adds and removes some smp_mb__after_atomic() calls to make the
> >> code correct even in the absence of x86's extra-strong atomics.
> >
> > The main point being that this better documents on which specific
> > ordering we rely.
> 
> Indeed.
> 
> >>               /*
> >> +              * Start remote flushes and then read tlb_gen.  As
> >> +              * above, the barrier synchronizes with
> >> +              * inc_mm_tlb_gen() like this:
> >> +              *
> >> +              * switch_mm_irqs_off():        flush request:
> >> +              *  cpumask_set_cpu(...);        inc_mm_tlb_gen();
> >> +              *  MB                           MB
> >> +              *  atomic64_read(.tlb_gen);     flush_tlb_others(mm_cpumask());
> >>                */
> >>               cpumask_set_cpu(cpu, mm_cpumask(next));
> >> +             smp_mb__after_atomic();
> >>               next_tlb_gen = atomic64_read(&next->context.tlb_gen);
> >>
> >>               choose_new_asid(next, next_tlb_gen, &new_asid, &need_flush);
> >
> > Arguably one could make a helper function of those few lines, not sure
> > it makes sense, but this duplication seems wasteful.
> >
> > So we either see the increment or the CPU set, but can not have neither.
> >
> > Should not arch_tlbbatch_add_mm() also have this same comment? It too
> > seems to increment and then read the mask.
> 
> Hmm.  There's already this comment in inc_mm_tlb_gen():
> 
>         /*
>          * Bump the generation count.  This also serves as a full barrier
>          * that synchronizes with switch_mm(): callers are required to order
>          * their read of mm_cpumask after their writes to the paging
>          * structures.
>          */
> 
> is that not adequate?

Yeah, I suppose so.

Thanks!