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

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Date:   Wed, 26 Jul 2017 06:52:06 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andy Lutomirski <luto@...nel.org>, 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 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?

FWIW, I have followup patches in the works to further de-deduplicate a
bunch of this code.  I wanted to get the main bits all landed first,
though.