linux-kernel - Re: [patch] oom: thaw threads if oom killed thread is frozen before deferring

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Date:	Thu, 29 Sep 2011 18:37:24 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Tejun Heo <htejun@...il.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [patch] oom: thaw threads if oom killed thread is frozen
	before deferring

On 09/29, Michal Hocko wrote:
>
> On Thu 29-09-11 14:05:17, Oleg Nesterov wrote:
>
> > But of course this can't help if freeze_task() is called later.
> > May be freezable() should check TIF_MEMDIE...
>
> Wouldn't it be easier to ignore try_to_freeze when fatal signals are
> pending in get_signal_to_deliver?

Oh, I don't think so. For what? This doesn't close other races, and
in fact the fatal_signal_pending() this patch adds is itself racy,
SIGKILL can come in between.

> --- a/kernel/freezer.c
> +++ b/kernel/freezer.c
> @@ -48,6 +48,11 @@ void refrigerator(void)
>  	current->flags |= PF_FREEZING;
>
>  	for (;;) {
> +		if (fatal_signal_pending(current)) {
> +			if (freezing(current) || frozen(current))
> +				thaw_process(current);

Ah, I didn't mean refrigerator() should check freezing/frozen.

I meant, oom_kill can do this before thaw thaw_process(), afaics
this should fix the particular race you described (but not others).

And. It is simply wrong to return from refrigerator() after we set
PF_FROZEN, this can fool try_to_freeze_tasks(). Sure, thaw_process()
from oom_kill is not nice too, but at least this is the special case,
we already have the problem.

Oleg.

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