On 02/07/2014 10:32 AM, Hans Verkuil wrote:
> 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,
Oops, that's without an '&' before dev->read_queue. I forgot to update my
patch before posting, sorry about that.
Hans
> + 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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