linux-kernel - Re: [RFC PATCH 4/7] x86: use exit_lazy_tlb rather than membarrier_mm_sync_core_before

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Date:   Fri, 17 Jul 2020 09:42:43 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Anton Blanchard <anton@...abs.org>,
        Arnd Bergmann <arnd@...db.de>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Andy Lutomirski <luto@...capital.net>,
        Andy Lutomirski <luto@...nel.org>, x86 <x86@...nel.org>
Subject: Re: [RFC PATCH 4/7] x86: use exit_lazy_tlb rather than
 membarrier_mm_sync_core_before_usermode

----- On Jul 16, 2020, at 7:26 PM, Nicholas Piggin npiggin@...il.com wrote:
[...]
> 
> membarrier does replace barrier instructions on remote CPUs, which do
> order accesses performed by the kernel on the user address space. So
> membarrier should too I guess.
> 
> Normal process context accesses like read(2) will do so because they
> don't get filtered out from IPIs, but kernel threads using the mm may
> not.

But it should not be an issue, because membarrier's ordering is only with respect
to submit and completion of io_uring requests, which are performed through
system calls from the context of user-space threads, which are called from the
right mm.

Thanks,

Mathieu


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