[PATCH] gpss: core: no waiters left behind on deregister

[PATCH] gpss: core: no waiters left behind on deregister

[Oliver Neukum]

If you deregister a device you need to wake up all waiters
as there will be no further wakeups.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
---
 drivers/gnss/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gnss/core.c b/drivers/gnss/core.c
index e6f94501cb28..0d13bd2cefd5 100644
--- a/drivers/gnss/core.c
+++ b/drivers/gnss/core.c
@@ -303,7 +303,7 @@ void gnss_deregister_device(struct gnss_device *gdev)
 	down_write(&gdev->rwsem);
 	gdev->disconnected = true;
 	if (gdev->count) {
-		wake_up_interruptible(&gdev->read_queue);
+		wake_up_interruptible_all(&gdev->read_queue);
 		gdev->ops->close(gdev);
 	}
 	up_write(&gdev->rwsem);
-- 
2.16.4

Re: [PATCH] gpss: core: no waiters left behind on deregister

[Johan Hovold]

On Mon, Jun 24, 2019 at 10:33:23AM +0200, Oliver Neukum wrote:
> If you deregister a device you need to wake up all waiters
> as there will be no further wakeups.
> 
> Signed-off-by: Oliver Neukum <oneukum@suse.com>
> ---
>  drivers/gnss/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gnss/core.c b/drivers/gnss/core.c
> index e6f94501cb28..0d13bd2cefd5 100644
> --- a/drivers/gnss/core.c
> +++ b/drivers/gnss/core.c
> @@ -303,7 +303,7 @@ void gnss_deregister_device(struct gnss_device *gdev)
>  	down_write(&gdev->rwsem);
>  	gdev->disconnected = true;
>  	if (gdev->count) {
> -		wake_up_interruptible(&gdev->read_queue);
> +		wake_up_interruptible_all(&gdev->read_queue);

GNSS core doesn't have any exclusive waiters, so no need to use use the
exclusive wake-up (all) interface.

>  		gdev->ops->close(gdev);
>  	}
>  	up_write(&gdev->rwsem);

Thanks,
Johan

Re: [PATCH] gpss: core: no waiters left behind on deregister

[Sergei Shtylyov]

Hello!

    The subject has gpss ISO gnss. You hardly meant the General Purpose
System Simulation. :-)

On 24.06.2019 11:33, Oliver Neukum wrote:

> If you deregister a device you need to wake up all waiters
> as there will be no further wakeups.
> 
> Signed-off-by: Oliver Neukum <oneukum@suse.com>
> ---
>   drivers/gnss/core.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gnss/core.c b/drivers/gnss/core.c
> index e6f94501cb28..0d13bd2cefd5 100644
> --- a/drivers/gnss/core.c
> +++ b/drivers/gnss/core.c
> @@ -303,7 +303,7 @@ void gnss_deregister_device(struct gnss_device *gdev)
>   	down_write(&gdev->rwsem);
>   	gdev->disconnected = true;
>   	if (gdev->count) {
> -		wake_up_interruptible(&gdev->read_queue);
> +		wake_up_interruptible_all(&gdev->read_queue);
>   		gdev->ops->close(gdev);
>   	}
>   	up_write(&gdev->rwsem);

MBR, Sergei

Re: [PATCH] gpss: core: no waiters left behind on deregister

[Oliver Neukum]

Am Dienstag, den 25.06.2019, 09:04 +0200 schrieb Johan Hovold:
> On Mon, Jun 24, 2019 at 10:33:23AM +0200, Oliver Neukum wrote:
> > If you deregister a device you need to wake up all waiters
> > as there will be no further wakeups.
> > 
> > Signed-off-by: Oliver Neukum <oneukum@suse.com>
> > ---
> >  drivers/gnss/core.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gnss/core.c b/drivers/gnss/core.c
> > index e6f94501cb28..0d13bd2cefd5 100644
> > --- a/drivers/gnss/core.c
> > +++ b/drivers/gnss/core.c
> > @@ -303,7 +303,7 @@ void gnss_deregister_device(struct gnss_device *gdev)
> >  	down_write(&gdev->rwsem);
> >  	gdev->disconnected = true;
> >  	if (gdev->count) {
> > -		wake_up_interruptible(&gdev->read_queue);
> > +		wake_up_interruptible_all(&gdev->read_queue);
> 
> GNSS core doesn't have any exclusive waiters, so no need to use use the
> exclusive wake-up (all) interface.

Well, yes, but that is the problem. In gnss_read() you drop the lock:

        mutex_lock(&gdev->read_mutex);

        while (kfifo_is_empty(&gdev->read_fifo)) {
                mutex_unlock(&gdev->read_mutex);

                if (gdev->disconnected)
                        return 0;

                if (file->f_flags & O_NONBLOCK)
                        return -EAGAIN;

That means that an arbitrary number of tasks can get here.

                ret = wait_event_interruptible(gdev->read_queue,
                                gdev->disconnected ||
                                !kfifo_is_empty(&gdev->read_fifo));

Meaning that an arbitrary number can be sleeping here. Yet in
gnss_deregister_device() you use a simple wake_up:

void gnss_deregister_device(struct gnss_device *gdev)

{

        down_write(&gdev->rwsem);
        gdev->disconnected = true;
        if (gdev->count) {
                wake_up_interruptible(&gdev->read_queue);


wake_up_interruptible() will wake up one waiting task. But after that
the device is gone. There will be no further events. The other tasks
will sleep forever.

	Regards
		Oliver

		

Re: [PATCH] gpss: core: no waiters left behind on deregister

[Johan Hovold]

On Wed, Jun 26, 2019 at 01:04:07PM +0200, Oliver Neukum wrote:
> Am Dienstag, den 25.06.2019, 09:04 +0200 schrieb Johan Hovold:
> > On Mon, Jun 24, 2019 at 10:33:23AM +0200, Oliver Neukum wrote:
> > > If you deregister a device you need to wake up all waiters
> > > as there will be no further wakeups.
> > > 
> > > Signed-off-by: Oliver Neukum <oneukum@suse.com>
> > > ---
> > >  drivers/gnss/core.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/gnss/core.c b/drivers/gnss/core.c
> > > index e6f94501cb28..0d13bd2cefd5 100644
> > > --- a/drivers/gnss/core.c
> > > +++ b/drivers/gnss/core.c
> > > @@ -303,7 +303,7 @@ void gnss_deregister_device(struct gnss_device *gdev)
> > >  	down_write(&gdev->rwsem);
> > >  	gdev->disconnected = true;
> > >  	if (gdev->count) {
> > > -		wake_up_interruptible(&gdev->read_queue);
> > > +		wake_up_interruptible_all(&gdev->read_queue);
> > 
> > GNSS core doesn't have any exclusive waiters, so no need to use use the
> > exclusive wake-up (all) interface.
> 
> Well, yes, but that is the problem. In gnss_read() you drop the lock:

> That means that an arbitrary number of tasks can get here.
> 
>                 ret = wait_event_interruptible(gdev->read_queue,
>                                 gdev->disconnected ||
>                                 !kfifo_is_empty(&gdev->read_fifo));
> 
> Meaning that an arbitrary number can be sleeping here.

I understand wait you're getting at, but I think your mistaken regarding
exclusive wait. Note that wait_event_interruptible() uses nonexclusive
wait.

> Yet in gnss_deregister_device() you use a simple wake_up:
> 
> void gnss_deregister_device(struct gnss_device *gdev)
> 
> {
> 
>         down_write(&gdev->rwsem);
>         gdev->disconnected = true;
>         if (gdev->count) {
>                 wake_up_interruptible(&gdev->read_queue);
> 
> 
> wake_up_interruptible() will wake up one waiting task. But after that
> the device is gone. There will be no further events. The other tasks
> will sleep forever.

No, wake_up_interruptible() will wake up all nonexclusive waiters,
which is all we care about here.

Johan

Re: [PATCH] gpss: core: no waiters left behind on deregister

[Oliver Neukum]

Am Mittwoch, den 26.06.2019, 13:41 +0200 schrieb Johan Hovold:
> On Wed, Jun 26, 2019 at 01:04:07PM +0200, Oliver Neukum wrote:
> > Am Dienstag, den 25.06.2019, 09:04 +0200 schrieb Johan Hovold:
> > > On Mon, Jun 24, 2019 at 10:33:23AM +0200, Oliver Neukum wrote:
> > > > If you deregister a device you need to wake up all waiters
> > > > as there will be no further wakeups.
> > > > 
> > > > Signed-off-by: Oliver Neukum <oneukum@suse.com>
> > > > ---
> > > >  drivers/gnss/core.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/drivers/gnss/core.c b/drivers/gnss/core.c
> > > > index e6f94501cb28..0d13bd2cefd5 100644
> > > > --- a/drivers/gnss/core.c
> > > > +++ b/drivers/gnss/core.c
> > > > @@ -303,7 +303,7 @@ void gnss_deregister_device(struct gnss_device *gdev)
> > > >  	down_write(&gdev->rwsem);
> > > >  	gdev->disconnected = true;
> > > >  	if (gdev->count) {
> > > > -		wake_up_interruptible(&gdev->read_queue);
> > > > +		wake_up_interruptible_all(&gdev->read_queue);
> > > 
> > > GNSS core doesn't have any exclusive waiters, so no need to use use the
> > > exclusive wake-up (all) interface.
> > 
> > Well, yes, but that is the problem. In gnss_read() you drop the lock:
> > That means that an arbitrary number of tasks can get here.
> > 
> >                 ret = wait_event_interruptible(gdev->read_queue,
> >                                 gdev->disconnected ||
> >                                 !kfifo_is_empty(&gdev->read_fifo));
> > 
> > Meaning that an arbitrary number can be sleeping here.
> 
> I understand wait you're getting at, but I think your mistaken regarding
> exclusive wait. Note that wait_event_interruptible() uses nonexclusive
> wait.
> 
> > Yet in gnss_deregister_device() you use a simple wake_up:
> > 
> > void gnss_deregister_device(struct gnss_device *gdev)
> > 
> > {
> > 
> >         down_write(&gdev->rwsem);
> >         gdev->disconnected = true;
> >         if (gdev->count) {
> >                 wake_up_interruptible(&gdev->read_queue);
> > 
> > 
> > wake_up_interruptible() will wake up one waiting task. But after that
> > the device is gone. There will be no further events. The other tasks
> > will sleep forever.
> 
> No, wake_up_interruptible() will wake up all nonexclusive waiters,
> which is all we care about here.

You are right and tracing this is hard.

	Regards & Sorry
		Oliver