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

[Oliver Neukum <oneukum@suse.com>]

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