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Date:   Mon, 13 Jul 2020 10:13:21 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Nicholas Piggin <npiggin@gmail.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Anton Blanchard <anton@ozlabs.org>,
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        Peter Zijlstra <peterz@infradead.org>, x86 <x86@kernel.org>
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Subject: Re: [RFC PATCH 4/7] x86: use exit_lazy_tlb rather than
 membarrier_mm_sync_core_before_usermode
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Thread-Topic: x86: use exit_lazy_tlb rather than membarrier_mm_sync_core_before_usermode
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----- On Jul 13, 2020, at 9:47 AM, Nicholas Piggin npiggin@gmail.com wrote:

> Excerpts from Nicholas Piggin's message of July 13, 2020 2:45 pm:
>> Excerpts from Andy Lutomirski's message of July 11, 2020 3:04 am:
>>> Also, as it stands, I can easily see in_irq() ceasing to promise to
>>> serialize.  There are older kernels for which it does not promise to
>>> serialize.  And I have plans to make it stop serializing in the
>>> nearish future.
>> 
>> You mean x86's return from interrupt? Sounds fun... you'll konw where to
>> update the membarrier sync code, at least :)
> 
> Oh, I should actually say Mathieu recently clarified a return from
> interrupt doesn't fundamentally need to serialize in order to support
> membarrier sync core.

Clarification to your statement:

Return from interrupt to kernel code does not need to be context serializing
as long as kernel serializes before returning to user-space.

However, return from interrupt to user-space needs to be context serializing.

Thanks,

Mathieu

> 
> https://lists.ozlabs.org/pipermail/linuxppc-dev/2020-July/214171.html
> 
> So you may not need to do anything more if you relaxed it.
> 
> Thanks,
> Nick

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com