diff mbox

[6/7,linux-next] wan: cosa: replace current->state by set_current_state()

Message ID 1424455977-21903-6-git-send-email-fabf@skynet.be
State Accepted, archived
Delegated to: David Miller
Headers show

Commit Message

Fabian Frederick Feb. 20, 2015, 6:12 p.m. UTC 
Use helper functions to access current->state.
Direct assignments are prone to races and therefore buggy.

current->state = TASK_RUNNING is replaced by __set_current_state()

Thanks to Peter Zijlstra for the exact definition of the problem.

Suggested-By: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
---
 drivers/net/wan/cosa.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

Comments

Jan Kasprzak Feb. 20, 2015, 6:26 p.m. UTC | #1 
Fabian Frederick wrote:
: Use helper functions to access current->state.
: Direct assignments are prone to races and therefore buggy.
: 
: current->state = TASK_RUNNING is replaced by __set_current_state()
: 
: Thanks to Peter Zijlstra for the exact definition of the problem.

	OK, thanks. Although I wonder whether real users of COSA (an ISA-based
WAN card) do exist.

Acked-By: Jan "Yenya" Kasprzak <kas@fi.muni.cz>

-Y.

: 
: Suggested-By: Peter Zijlstra <peterz@infradead.org>
: Signed-off-by: Fabian Frederick <fabf@skynet.be>
: ---
:  drivers/net/wan/cosa.c | 12 ++++++------
:  1 file changed, 6 insertions(+), 6 deletions(-)
: 
: diff --git a/drivers/net/wan/cosa.c b/drivers/net/wan/cosa.c
: index 83c39e2..88d121d 100644
: --- a/drivers/net/wan/cosa.c
: +++ b/drivers/net/wan/cosa.c
: @@ -806,21 +806,21 @@ static ssize_t cosa_read(struct file *file,
:  	spin_lock_irqsave(&cosa->lock, flags);
:  	add_wait_queue(&chan->rxwaitq, &wait);
:  	while (!chan->rx_status) {
: -		current->state = TASK_INTERRUPTIBLE;
: +		set_current_state(TASK_INTERRUPTIBLE);
:  		spin_unlock_irqrestore(&cosa->lock, flags);
:  		schedule();
:  		spin_lock_irqsave(&cosa->lock, flags);
:  		if (signal_pending(current) && chan->rx_status == 0) {
:  			chan->rx_status = 1;
:  			remove_wait_queue(&chan->rxwaitq, &wait);
: -			current->state = TASK_RUNNING;
: +			__set_current_state(TASK_RUNNING);
:  			spin_unlock_irqrestore(&cosa->lock, flags);
:  			mutex_unlock(&chan->rlock);
:  			return -ERESTARTSYS;
:  		}
:  	}
:  	remove_wait_queue(&chan->rxwaitq, &wait);
: -	current->state = TASK_RUNNING;
: +	__set_current_state(TASK_RUNNING);
:  	kbuf = chan->rxdata;
:  	count = chan->rxsize;
:  	spin_unlock_irqrestore(&cosa->lock, flags);
: @@ -890,14 +890,14 @@ static ssize_t cosa_write(struct file *file,
:  	spin_lock_irqsave(&cosa->lock, flags);
:  	add_wait_queue(&chan->txwaitq, &wait);
:  	while (!chan->tx_status) {
: -		current->state = TASK_INTERRUPTIBLE;
: +		set_current_state(TASK_INTERRUPTIBLE);
:  		spin_unlock_irqrestore(&cosa->lock, flags);
:  		schedule();
:  		spin_lock_irqsave(&cosa->lock, flags);
:  		if (signal_pending(current) && chan->tx_status == 0) {
:  			chan->tx_status = 1;
:  			remove_wait_queue(&chan->txwaitq, &wait);
: -			current->state = TASK_RUNNING;
: +			__set_current_state(TASK_RUNNING);
:  			chan->tx_status = 1;
:  			spin_unlock_irqrestore(&cosa->lock, flags);
:  			up(&chan->wsem);
: @@ -905,7 +905,7 @@ static ssize_t cosa_write(struct file *file,
:  		}
:  	}
:  	remove_wait_queue(&chan->txwaitq, &wait);
: -	current->state = TASK_RUNNING;
: +	__set_current_state(TASK_RUNNING);
:  	up(&chan->wsem);
:  	spin_unlock_irqrestore(&cosa->lock, flags);
:  	kfree(kbuf);
: -- 
: 2.1.0
Sergei Shtylyov Feb. 20, 2015, 6:34 p.m. UTC | #2 
Hello.

On 02/20/2015 09:12 PM, Fabian Frederick wrote:

> Use helper functions to access current->state.
> Direct assignments are prone to races and therefore buggy.

> current->state = TASK_RUNNING is replaced by __set_current_state()

    You sometimes use __set_current_state() and sometimes set_current_state().

> Thanks to Peter Zijlstra for the exact definition of the problem.

> Suggested-By: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Fabian Frederick <fabf@skynet.be>

[...]

WBR, Sergei

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Fabian Frederick Feb. 20, 2015, 6:51 p.m. UTC | #3 
> On 20 February 2015 at 19:34 Sergei Shtylyov
> <sergei.shtylyov@cogentembedded.com> wrote:
>
>
> Hello.
>
> On 02/20/2015 09:12 PM, Fabian Frederick wrote:
>
> > Use helper functions to access current->state.
> > Direct assignments are prone to races and therefore buggy.
>
> > current->state = TASK_RUNNING is replaced by __set_current_state()
>
>     You sometimes use __set_current_state() and sometimes set_current_state().
Hello Sergei,

Peter suggested to use __set_current_state() for TASK_RUNNING :
http://marc.info/?l=linux-kernel&m=142442259719216&w=2

Regards,
Fabian

>
> > Thanks to Peter Zijlstra for the exact definition of the problem.
>
> > Suggested-By: Peter Zijlstra <peterz@infradead.org>
> > Signed-off-by: Fabian Frederick <fabf@skynet.be>
>
> [...]
>
> WBR, Sergei
>
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Peter Zijlstra Feb. 20, 2015, 6:58 p.m. UTC | #4 
On Fri, Feb 20, 2015 at 09:34:28PM +0300, Sergei Shtylyov wrote:
> Hello.
> 
> On 02/20/2015 09:12 PM, Fabian Frederick wrote:
> 
> >Use helper functions to access current->state.
> >Direct assignments are prone to races and therefore buggy.
> 
> >current->state = TASK_RUNNING is replaced by __set_current_state()
> 
>    You sometimes use __set_current_state() and sometimes set_current_state().

It depends on which state; setting yourself TASK_RUNNING is free of
wakeup races -- you're already running after all, so it can safely use
__set_current_state().

Setting a blocking state otoh needs set_current_state() which issues a
full memory barriers with the store (critically in this case,
effectively after the store) such that it orders the state store with a
subsequent load in the condition check if it really needs to go to
sleep.


In full:

	current->state = TASK_UNINTERRUPTIBLE;		wait = false;
	smp_mb();					smp_wmb();
	if (wait)					p->state = TASK_RUNNING;
		schedule();

Without that smp_mb(); the following order is possible:

	if (wait)
							wait = false;
							smp_wmb();
							p->state = TASK_RUNNING;
	current->state = TASK_UNINTERRUPTIBLE;
		schedule();

And we'll wait forever more..

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Sergei Shtylyov Feb. 20, 2015, 7:15 p.m. UTC | #5 
On 02/20/2015 09:51 PM, Fabian Frederick wrote:

>>> Use helper functions to access current->state.
>>> Direct assignments are prone to races and therefore buggy.

>>> current->state = TASK_RUNNING is replaced by __set_current_state()

>>       You sometimes use __set_current_state() and sometimes set_current_state().

> Hello Sergei,

> Peter suggested to use __set_current_state() for TASK_RUNNING :
> http://marc.info/?l=linux-kernel&m=142442259719216&w=2

    I didn't even question your decisions, I (like Peter) just wanted a more 
coherent change-log. Thanks to Peter for the explanations though. :-)

> Regards,
> Fabian

>>> Thanks to Peter Zijlstra for the exact definition of the problem.

>>> Suggested-By: Peter Zijlstra <peterz@infradead.org>
>>> Signed-off-by: Fabian Frederick <fabf@skynet.be>

>> [...]

WBR, Sergei

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Fabian Frederick Feb. 21, 2015, 7:42 a.m. UTC | #6 
> On 20 February 2015 at 19:58 Peter Zijlstra <peterz@infradead.org> wrote:
>
>
> On Fri, Feb 20, 2015 at 09:34:28PM +0300, Sergei Shtylyov wrote:
> > Hello.
> >
> > On 02/20/2015 09:12 PM, Fabian Frederick wrote:
> >
> > >Use helper functions to access current->state.
> > >Direct assignments are prone to races and therefore buggy.
> >
> > >current->state = TASK_RUNNING is replaced by __set_current_state()
> >
> >    You sometimes use __set_current_state() and sometimes
> >set_current_state().
>
> It depends on which state; setting yourself TASK_RUNNING is free of
> wakeup races -- you're already running after all, so it can safely use
> __set_current_state().
>
> Setting a blocking state otoh needs set_current_state() which issues a
> full memory barriers with the store (critically in this case,
> effectively after the store) such that it orders the state store with a
> subsequent load in the condition check if it really needs to go to
> sleep.
>
>
> In full:
>
>       current->state = TASK_UNINTERRUPTIBLE;          wait = false;
>       smp_mb();                                       smp_wmb();
>       if (wait)                                       p->state = TASK_RUNNING;
>               schedule();
>
> Without that smp_mb(); the following order is possible:
>
>       if (wait)
>                                                       wait = false;
>                                                       smp_wmb();
>                                                       p->state = TASK_RUNNING;
>       current->state = TASK_UNINTERRUPTIBLE;
>               schedule();
>
> And we'll wait forever more..
Do I have to add more comments in changelogs or is it OK for you ?
Maybe something like:
"
current->state = TASK_RUNNING can safely be converted to __set_current_state()
as we're already in that state. Other assignments are converted to
set_current_state() (which uses set_mb()).

"
Regards,
Fabian
>
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David Miller Feb. 22, 2015, 8:25 p.m. UTC | #7 
From: Fabian Frederick <fabf@skynet.be>
Date: Fri, 20 Feb 2015 19:12:56 +0100

> Use helper functions to access current->state.
> Direct assignments are prone to races and therefore buggy.
> 
> current->state = TASK_RUNNING is replaced by __set_current_state()
> 
> Thanks to Peter Zijlstra for the exact definition of the problem.
> 
> Suggested-By: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Fabian Frederick <fabf@skynet.be>

Applied.
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diff mbox

Patch

diff --git a/drivers/net/wan/cosa.c b/drivers/net/wan/cosa.c
index 83c39e2..88d121d 100644
--- a/drivers/net/wan/cosa.c
+++ b/drivers/net/wan/cosa.c
@@ -806,21 +806,21 @@  static ssize_t cosa_read(struct file *file,
 	spin_lock_irqsave(&cosa->lock, flags);
 	add_wait_queue(&chan->rxwaitq, &wait);
 	while (!chan->rx_status) {
-		current->state = TASK_INTERRUPTIBLE;
+		set_current_state(TASK_INTERRUPTIBLE);
 		spin_unlock_irqrestore(&cosa->lock, flags);
 		schedule();
 		spin_lock_irqsave(&cosa->lock, flags);
 		if (signal_pending(current) && chan->rx_status == 0) {
 			chan->rx_status = 1;
 			remove_wait_queue(&chan->rxwaitq, &wait);
-			current->state = TASK_RUNNING;
+			__set_current_state(TASK_RUNNING);
 			spin_unlock_irqrestore(&cosa->lock, flags);
 			mutex_unlock(&chan->rlock);
 			return -ERESTARTSYS;
 		}
 	}
 	remove_wait_queue(&chan->rxwaitq, &wait);
-	current->state = TASK_RUNNING;
+	__set_current_state(TASK_RUNNING);
 	kbuf = chan->rxdata;
 	count = chan->rxsize;
 	spin_unlock_irqrestore(&cosa->lock, flags);
@@ -890,14 +890,14 @@  static ssize_t cosa_write(struct file *file,
 	spin_lock_irqsave(&cosa->lock, flags);
 	add_wait_queue(&chan->txwaitq, &wait);
 	while (!chan->tx_status) {
-		current->state = TASK_INTERRUPTIBLE;
+		set_current_state(TASK_INTERRUPTIBLE);
 		spin_unlock_irqrestore(&cosa->lock, flags);
 		schedule();
 		spin_lock_irqsave(&cosa->lock, flags);
 		if (signal_pending(current) && chan->tx_status == 0) {
 			chan->tx_status = 1;
 			remove_wait_queue(&chan->txwaitq, &wait);
-			current->state = TASK_RUNNING;
+			__set_current_state(TASK_RUNNING);
 			chan->tx_status = 1;
 			spin_unlock_irqrestore(&cosa->lock, flags);
 			up(&chan->wsem);
@@ -905,7 +905,7 @@  static ssize_t cosa_write(struct file *file,
 		}
 	}
 	remove_wait_queue(&chan->txwaitq, &wait);
-	current->state = TASK_RUNNING;
+	__set_current_state(TASK_RUNNING);
 	up(&chan->wsem);
 	spin_unlock_irqrestore(&cosa->lock, flags);
 	kfree(kbuf);