Re: [PATCH, RFC 07/30] [media] radio-cadet: avoid interruptible_sleep_on race

[Hans Verkuil <hverkuil@xs4all.nl>]

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@cisco.com>
> 
> 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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