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Subject: Re: [v2 PATCH] mm: introduce arg_lock to protect arg_start|end and
 env_start|end in mm_struct
To:     Cyrill Gorcunov <gorcunov@gmail.com>,
        Matthew Wilcox <willy@infradead.org>
Cc:     Yang Shi <yang.shi@linux.alibaba.com>, adobriyan@gmail.com,
        mhocko@kernel.org, mguzik@redhat.com, akpm@linux-foundation.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
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From:   Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Message-ID: <0bfa8943-a2fe-b0ab-99a2-347094a2bcec@i-love.sakura.ne.jp>
Date:   Tue, 27 Mar 2018 06:10:09 +0900
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On 2018/03/27 4:21, Cyrill Gorcunov wrote:
> That said I think using read-lock here would be a bug.

If I understand correctly, the caller can't set both fields atomically, for
prctl() does not receive both fields at one call.

  prctl(PR_SET_MM, PR_SET_MM_ARG_START xor PR_SET_MM_ARG_END xor PR_SET_MM_ENV_START xor PR_SET_MM_ENV_END, new value, 0, 0);

Then, I wonder whether reading arg_start|end and env_start|end atomically makes
sense. Just retry reading if arg_start > env_end or env_start > env_end is fine?