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Date:	Mon, 11 Aug 2014 15:18:28 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Cong Wang <xiyou.wangcong@...il.com>
Cc:	linux-kernel@...r.kernel.org, David Rientjes <rientjes@...gle.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>, Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [Patch] freezer: check OOM kill signal while being frozen

On Fri 08-08-14 17:46:38, Cong Wang wrote:
> There is a race condition between OOM killer and freezer when
> they try to operate on the same process, something like below:
> 
> 	Process A	Process B		Process C
> trigger oom
> B=oom_scan_process_thread()
> 					cgroup_freezer_freeze(B)
> 			...
> 			try_to_freeze()
> 			stay in D state
> oom_kill_process(B)
> 
> In this case, process A triggers OOM and kernel selects process B
> as the victim, right before being killed process B was frozen by
> process C therefore went to D state, then kernel sent SIGKILL but
> it is already too late as process B will never care about pending
> signals any more.

OK, so the system/memcg is still OOM and a new allocation/charge
would trigger killer again, right? Then oom_scan_process_thread sees
TIF_MEMDIE frozen task and thaw it so it can go away and die. So this
shouldn't be a permanent state. Or am I missing something?

> Fix this straightly by checking fatal pending signal
> from OOM killer, so that the frozen process will recover itself
> and then be killed finally.
> 
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: Michal Hocko <mhocko@...e.cz>
> Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>
> Cc: Tejun Heo <tj@...nel.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
> ---
> diff --git a/kernel/freezer.c b/kernel/freezer.c
> index aa6a8aa..c6d189d 100644
> --- a/kernel/freezer.c
> +++ b/kernel/freezer.c
> @@ -68,7 +68,9 @@ bool __refrigerator(bool check_kthr_stop)
>  		spin_lock_irq(&freezer_lock);
>  		current->flags |= PF_FROZEN;
>  		if (!freezing(current) ||
> -		    (check_kthr_stop && kthread_should_stop()))
> +		    (check_kthr_stop && kthread_should_stop()) ||
> +		    (test_tsk_thread_flag(current, TIF_MEMDIE) &&
> +			 fatal_signal_pending(current)))
>  			current->flags &= ~PF_FROZEN;
>  		spin_unlock_irq(&freezer_lock);
>  

-- 
Michal Hocko
SUSE Labs
--
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