linux-kernel - Re: [PATCH 1/6] proc, oom: drop bogus task

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Message-ID: <20160530174324.GA25382@redhat.com>
Date:	Mon, 30 May 2016 19:43:24 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	linux-mm@...ck.org,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	David Rientjes <rientjes@...gle.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 1/6] proc, oom: drop bogus task_lock and mm check

On 05/30, Michal Hocko wrote:
>
> both oom_adj_write and oom_score_adj_write are using task_lock,
> check for task->mm and fail if it is NULL. This is not needed because
> the oom_score_adj is per signal struct so we do not need mm at all.
> The code has been introduced by 3d5992d2ac7d ("oom: add per-mm oom
> disable count") but we do not do per-mm oom disable since c9f01245b6a7
> ("oom: remove oom_disable_count").
>
> The task->mm check is even not correct because the current thread might
> have exited but the thread group might be still alive - e.g. thread
> group leader would lead that echo $VAL > /proc/pid/oom_score_adj would
> always fail with EINVAL while /proc/pid/task/$other_tid/oom_score_adj
> would succeed. This is unexpected at best.
>
> Remove the lock along with the check to fix the unexpected behavior
> and also because there is not real need for the lock in the first place.

ACK

and we should also remove lock_task_sighand(). as for oom_adj_read() and
oom_score_adj_read() we can just remove it right now; it was previously
needed to ensure the task->signal != NULL, today this is always true.

Oleg.