linux-kernel - Re: [PATCH v4] mm, oom: Fix race when selecting process to kill

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Message-ID: <20131112200156.GA9820@redhat.com>
Date:	Tue, 12 Nov 2013 21:01:56 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Sameer Nanda <snanda@...omium.org>
Cc:	akpm@...ux-foundation.org, mhocko@...e.cz, rientjes@...gle.com,
	hannes@...xchg.org, rusty@...tcorp.com.au, semenzato@...gle.com,
	murzin.v@...il.com, dserrg@...il.com, msb@...omium.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] mm, oom: Fix race when selecting process to kill

On 11/11, Sameer Nanda wrote:
>
> The selection of the process to be killed happens in two spots:
> first in select_bad_process and then a further refinement by
> looking for child processes in oom_kill_process. Since this is
> a two step process, it is possible that the process selected by
> select_bad_process may get a SIGKILL just before oom_kill_process
> executes. If this were to happen, __unhash_process deletes this
> process from the thread_group list. This results in oom_kill_process
> getting stuck in an infinite loop when traversing the thread_group
> list of the selected process.
>
> Fix this race by adding a pid_alive check for the selected process
> with tasklist_lock held in oom_kill_process.

OK, looks correct to me. Thanks.


Yes, this is a step backwards, hopefully we will revert this patch soon.
I am starting to think something like while_each_thread_lame_but_safe()
makes sense before we really fix this nasty (and afaics not simple)
problem with with while_each_thread() (which should die).

Oleg.

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