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Subject: Re: [PATCH 5/8] membarrier, kthread: Use _ONCE accessors for task->mm
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     x86 <x86@kernel.org>, Dave Hansen <dave.hansen@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>
References: <cover.1623813516.git.luto@kernel.org>
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 <987970694.9105.1623866911729.JavaMail.zimbra@efficios.com>
From:   Andy Lutomirski <luto@kernel.org>
Message-ID: <7e411333-2fbc-6254-671f-bf2cba486bcd@kernel.org>
Date:   Wed, 16 Jun 2021 11:45:44 -0700
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On 6/16/21 11:08 AM, Mathieu Desnoyers wrote:
> ----- On Jun 15, 2021, at 11:21 PM, Andy Lutomirski luto@kernel.org wrote:
> 
>> membarrier reads cpu_rq(remote cpu)->curr->mm without locking.  Use
>> READ_ONCE() and WRITE_ONCE() to remove the data races.
> 
> I notice that kernel/exit.c:exit_mm() also has:
> 
>         current->mm = NULL;
> 
> I suspect you may want to add a WRITE_ONCE() there as well ?

Good catch.  I was thinking that exit_mm() couldn't execute concurrently
with membarrier(), but that's wrong.

--Andy