Hi Arnd!
On 01/17/2014 03:28 PM, Arnd Bergmann wrote:
> 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.
I don't see it. I propose this patch:
Signed-off-by: Hans Verkuil <hans.verkuil@xxxxxxxxx>
diff --git a/drivers/media/radio/radio-cadet.c b/drivers/media/radio/radio-cadet.c
index 545c04c..2f658c6 100644
--- a/drivers/media/radio/radio-cadet.c
+++ b/drivers/media/radio/radio-cadet.c
@@ -327,13 +327,15 @@ static ssize_t cadet_read(struct file *file, char __user *data, size_t count, lo
mutex_lock(&dev->lock);
if (dev->rdsstat == 0)
cadet_start_rds(dev);
- if (dev->rdsin == dev->rdsout) {
+ while (dev->rdsin == dev->rdsout) {
if (file->f_flags & O_NONBLOCK) {
i = -EWOULDBLOCK;
goto unlock;
}
mutex_unlock(&dev->lock);
- interruptible_sleep_on(&dev->read_queue);
+ if (wait_event_interruptible(&dev->read_queue,
+ dev->rdsin != dev->rdsout))
+ return -EINTR;
mutex_lock(&dev->lock);
}
while (i < count && dev->rdsin != dev->rdsout)
Tested with my radio-cadet card.
This looks good to me. If I am still missing something, let me know!
Regards,
Hans
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