On Friday 17 January 2014, Hans Verkuil wrote:
> > @@ -323,25 +324,32 @@ static ssize_t cadet_read(struct file *file, char
> > __user *data, size_t count, lo
> > struct cadet *dev = video_drvdata(file);
> > unsigned char readbuf[RDS_BUFFER];
> > int i = 0;
> > + DEFINE_WAIT(wait);
> >
> > mutex_lock(&dev->lock);
> > if (dev->rdsstat == 0)
> > cadet_start_rds(dev);
> > - if (dev->rdsin == dev->rdsout) {
> > + while (1) {
> > + prepare_to_wait(&dev->read_queue, &wait, TASK_INTERRUPTIBLE);
> > + if (dev->rdsin != dev->rdsout)
> > + break;
> > +
> > if (file->f_flags & O_NONBLOCK) {
> > i = -EWOULDBLOCK;
> > goto unlock;
> > }
> > mutex_unlock(&dev->lock);
> > - interruptible_sleep_on(&dev->read_queue);
> > + schedule();
> > mutex_lock(&dev->lock);
> > }
> > +
>
> This seems overly complicated. Isn't it enough to replace
> interruptible_sleep_on
> by 'wait_event_interruptible(&dev->read_queue, dev->rdsin != dev->rdsout);'?
>
> Or am I missing something subtle?
The existing code sleeps with &dev->lock released because the cadet_handler()
function needs to grab (and release) the same lock before it can wake up
the reader thread.
Doing the simple wait_event_interruptible() would result in a deadlock here.
Arnd
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