[PATCH] fix refrigerator() vs thaw_process() race

[PATCH] fix refrigerator() vs thaw_process() race

[Oleg Nesterov]

refrigerator() can miss a wakeup, "wait event" loop needs a proper memory
ordering.

Signed-off-by: Oleg Nesterov <oleg@tv-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);
 
-	while (frozen(current)) {
-		current->state = TASK_UNINTERRUPTIBLE;
+	for (;;) {
+		set_current_state(TASK_UNINTERRUPTIBLE);
+		if (!frozen(current))
+			break;
 		schedule();
 	}
 	pr_debug("%s left refrigerator\n", current->comm);

Re: [PATCH] fix refrigerator() vs thaw_process() race

[Rafael J. Wysocki]

On Sunday, 18 February 2007 23:17, Oleg Nesterov wrote:
> refrigerator() can miss a wakeup, "wait event" loop needs a proper memory
> ordering.
> 
> Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

ACK 

> --- 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);
>  
> -	while (frozen(current)) {
> -		current->state = TASK_UNINTERRUPTIBLE;
> +	for (;;) {
> +		set_current_state(TASK_UNINTERRUPTIBLE);
> +		if (!frozen(current))
> +			break;
>  		schedule();
>  	}
>  	pr_debug("%s left refrigerator\n", current->comm);
> 
> 
> 

-- 
If you don't have the time to read,
you don't have the time or the tools to write.
		- Stephen King

Re: [PATCH] fix refrigerator() vs thaw_process() race

[Pavel Machek]

Hi!

> refrigerator() can miss a wakeup, "wait event" loop needs a proper memory
> ordering.
> 
> Signed-off-by: Oleg Nesterov <oleg@tv-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?

								Pavel

>  		schedule();
>  	}
>  	pr_debug("%s left refrigerator\n", current->comm);

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

Re: [PATCH] fix refrigerator() vs thaw_process() race

[Oleg Nesterov]

On 02/19, Pavel Machek wrote:
> 
> > refrigerator() can miss a wakeup, "wait event" loop needs a proper memory
> > ordering.
> > 
> > Signed-off-by: Oleg Nesterov <oleg@tv-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).

Oleg.

Re: [PATCH] fix refrigerator() vs thaw_process() race

[Pavel Machek]

Hi!

> > > refrigerator() can miss a wakeup, "wait event" loop needs a proper memory
> > > ordering.
> > > 
> > > Signed-off-by: Oleg Nesterov <oleg@tv-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