linux-kernel - Re: [PATCH v2 05/10] x86/mm: Rework lazy TLB mode and TLB freshness tracking

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Date:   Wed, 14 Jun 2017 15:42:59 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
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>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Banman <abanman@....com>, Mike Travis <travis@....com>,
        Dimitri Sivanich <sivanich@....com>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [PATCH v2 05/10] x86/mm: Rework lazy TLB mode and TLB freshness tracking

On Wed, Jun 14, 2017 at 3:33 PM, Dave Hansen <dave.hansen@...el.com> wrote:
> On 06/13/2017 09:56 PM, Andy Lutomirski wrote:
>> -     if (cpumask_test_cpu(cpu, &batch->cpumask))
>> +     if (cpumask_test_cpu(cpu, &batch->cpumask)) {
>> +             local_irq_disable();
>>               flush_tlb_func_local(&info, TLB_LOCAL_SHOOTDOWN);
>> +             local_irq_enable();
>> +     }
>> +
>
> Could you talk a little about why this needs to be local_irq_disable()
> and not preempt_disable()?  Is it about the case where somebody is
> trying to call flush_tlb_func_*() from an interrupt handler?

It's to prevent flush_tlb_func_local() and flush_tlb_func_remote()
from being run concurrently, which would cause flush_tlb_func_common()
to be reentered.  Either we'd need to be very careful in
flush_tlb_func_common() to avoid races if this happened, or we could
just disable interrupts around flush_tlb_func_local().  The latter is
fast and easy.

--Andy