linux-kernel - Re: [RFC PATCH 1/2] Fix PF_NOFREEZE and freezeable race

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Date:	Fri, 20 Apr 2007 22:02:08 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	ego@...ibm.com, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, mingo@...e.hu, vatsa@...ibm.com,
	paulmck@...ibm.com, pavel@....cz
Subject: Re: [RFC PATCH 1/2] Fix PF_NOFREEZE and freezeable race

On 04/19, Rafael J. Wysocki wrote:
> 
> On Thursday, 19 April 2007 14:02, Gautham R Shenoy wrote:
> > This patch fixes the race pointed out by Oleg Nesterov.
> > 
> > * Freezer marks a thread as freezeable. 
> > * The thread now marks itself PF_NOFREEZE causing it to
> >   freeze on calling try_to_freeze(). Thus the task is frozen, even though
> >   it doesn't want to.
> > * Subsequent thaw_processes() will also fail to thaw the task since it is 
> >   marked PF_NOFREEZE.
> > 
> > Avoid this problem by checking the current task's PF_NOFREEZE status in the 
> > refrigerator before marking current as frozen.
> > 
> > Signed-off-by: Gautham R Shenoy <ego@...ibm.com>
> 
> Looks good, although I'm not sure if we don't need to call recalc_sigpending()
> for tasks that turn out to be PF_NOFREEZE.

I agree, we should clear TIF_SIGPENDING. It is not so critical for user-space
tasks, but for the kernel thread it may remain pending forever, causing subtle
failures.

Gautham, isn't it possible to make a more simpler patch ? Just add PF_NOFREEZE
check to frozen_process,

	static inline void frozen_process(struct task_struct *p)
	{
		if (!unlikely(current->flags & PF_NOFREEZE)) {
			p->flags |= PF_FROZEN;
			wmb();
		}
		clear_tsk_thread_flag(p, TIF_FREEZE);
	}

No?

Oleg.

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