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

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Message-ID: <20110929120517.GA10587@redhat.com>
Date:	Thu, 29 Sep 2011 14:05:17 +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:
>
> --- a/kernel/freezer.c
> +++ b/kernel/freezer.c
> @@ -48,6 +48,10 @@ void refrigerator(void)
>  	current->flags |= PF_FREEZING;
>  
>  	for (;;) {
> +		if (fatal_signal_pending(current)) {
> +			current->flags &= ~PF_FROZEN;

We can't do this.

If PF_FROZEN was set, we must not modify current->flags, this can
race with, say, thaw_process().

OK, we can take task_lock(), but this doesn't close other races.
Say, a SIGKILL'ed task can do try_to_freeze(). Perhaps we should
simply call thaw_process() unconditionally, this also clears
TIF_FREEZE. Or check freezing() || frozen(). Afacis this solves
the race you described.

But of course this can't help if freeze_task() is called later.
May be freezable() should check TIF_MEMDIE...

Oleg.

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