linux-kernel - Re: [PATCH] fix refrigerator() vs thaw

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Message-ID: <20070219224725.GF21199@elf.ucw.cz>
Date:	Mon, 19 Feb 2007 23:47:25 +0100
From:	Pavel Machek <pavel@....cz>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fix refrigerator() vs thaw_process() race

Hi!

> > > refrigerator() can miss a wakeup, "wait event" loop needs a proper memory
> > > ordering.
> > > 
> > > Signed-off-by: Oleg Nesterov <oleg@...sign.ru>
> > > 
> > > --- WQ/kernel/power/process.c~WAKE	2007-02-18 22:56:49.000000000 +0300
> > > +++ WQ/kernel/power/process.c	2007-02-19 01:04:26.000000000 +0300
> > > @@ -46,8 +46,10 @@ void refrigerator(void)
> > >  	recalc_sigpending(); /* We sent fake signal, clean it up */
> > >  	spin_unlock_irq(&current->sighand->siglock);
> > >  
> > > -		current->state = TASK_UNINTERRUPTIBLE;
> > > +		set_current_state(TASK_UNINTERRUPTIBLE);
> > 
> > 
> > Looks okay to me... but this one liner would be exactly as effective,
> > right?
> 
> I think no, with this one liner we have
> 
> 	while (frozen(current)) {
> 		// ------ WINDOW ------------
> 		set_current_state(TASK_UNINTERRUPTIBLE);
> 		schedule();
> 	}
> 
> What if thaw_process() happens in the window above?
> 
> We need the barrier exactly because LOAD (check condition) should not
> come before STORE (set task->state).

Aha, ok :-). You already have my ACK ;-).
									Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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