Re: [PATCH] x86/mm: Add barriers and document switch_mm()-vs-flush synchronization follow-up

From	Nadav Amit <nadav.amit@gmail.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH] x86/mm: Add barriers and document switch_mm()-vs-flush synchronization follow-up
Date	2016-08-03 02:50 +0200
Message-ID	<s1SxH-3vo-7@gated-at.bofh.it> (permalink)
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Rafael Aquini <aquini@redhat.com> wrote:

> On Tue, Aug 02, 2016 at 03:27:06PM -0700, Nadav Amit wrote:
>> Rafael Aquini <aquini@redhat.com> wrote:
>> 
>>> While backporting 71b3c126e611 ("x86/mm: Add barriers and document switch_mm()-vs-flush synchronization")
>>> we stumbled across a possibly missing barrier at flush_tlb_page().
>> 
>> I too noticed it and submitted a similar patch that never got a response [1].
> 
> As far as I understood Andy's rationale for the original patch you need
> a full memory barrier there in flush_tlb_page to get that cache-eviction
> race sorted out.

I am completely ok with your fix (except for the missing barrier in
set_tlb_ubc_flush_pending() ). However, I think mine should suffice. As far as
I saw, an atomic operation preceded every invocation of flush_tlb_page(). I
was afraid someone would send me to measure the patch performance impact so I
looked for one with the least impact.

See Intel SDM 8.2.2 "Memory Ordering in P6 and More Recent Processor Families"
for the reasoning behind smp_mb__after_atomic() . The result of an atomic
operation followed by smp_mb__after_atomic should be identical to smp_mb().

Regards,
Nadav

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[PATCH] x86/mm: Add barriers and document switch_mm()-vs-flush synchronization follow-up Rafael Aquini <aquini@redhat.com> - 2016-08-02 17:50 +0200
  Re: [PATCH] x86/mm: Add barriers and document switch_mm()-vs-flush synchronization follow-up Nadav Amit <nadav.amit@gmail.com> - 2016-08-03 00:30 +0200
    Re: [PATCH] x86/mm: Add barriers and document switch_mm()-vs-flush  synchronization follow-up Andy Lutomirski <luto@amacapital.net> - 2016-08-03 00:50 +0200
    Re: [PATCH] x86/mm: Add barriers and document switch_mm()-vs-flush  synchronization follow-up Rafael Aquini <aquini@redhat.com> - 2016-08-03 01:20 +0200
      Re: [PATCH] x86/mm: Add barriers and document switch_mm()-vs-flush synchronization follow-up Nadav Amit <nadav.amit@gmail.com> - 2016-08-03 02:50 +0200

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