[alsa-devel] [PATCH] ALSA: control: add .flush operation to release blocked operation

[Takashi Iwai]

On Sat, 25 Jul 2015 13:54:00 +0200,
Takashi Sakamoto wrote:
> 
> ALSA control character devices doesn't release blocking state for
> read(2) operations when a file descriptor corresponding to the character
> device is closed by close(2). This is due to a blocking loop of
> snd_ctl_read() has no breaking point for this event.
> 
> Additionally, the read(2) operation reports EBADFD when executed in a
> state of unsubscription. On the other hand, this operation continue
> to be blocked after unsubscribe operation. In this case, the read(2)
> operation should return with EBADFD.
> 
> Furthermore, poll(2) operation for the character device can report
> some events after close(2).
> 
> This commit solves these issues, by adding flush file operation.
> The file descriptor becomes unsubscribed state after close(2) operation.
> Then, read(2) and poll(2) operations return with expected results.
> 
> Signed-off-by: Takashi Sakamoto <o-takashi at sakamocchi.jp>

Thanks for spotting out the problem and the patch.  I guess your patch
would work in most cases correctly.  One thing I'm not sure is the
case where the file descriptor is copied.  Is the flush op called only
if one of the multiple processes is closed, or is it after all fps
closed?  In the former case, this might be a problem.

BTW, a minor nitpick: please omit the explicit 0 or 1 comparison for
subscribed field.  It's a (kind of) boolean flag, so a form like
    if (ctl->subscribed)

or
    if (!ctl->subscribed)

is preferred.


thanks,

Takashi


> ---
>  sound/core/control.c | 38 +++++++++++++++++++++++++++++++-------
>  1 file changed, 31 insertions(+), 7 deletions(-)
> 
> diff --git a/sound/core/control.c b/sound/core/control.c
> index 196a6fe..0337c16 100644
> --- a/sound/core/control.c
> +++ b/sound/core/control.c
> @@ -113,6 +113,21 @@ static void snd_ctl_empty_read_queue(struct snd_ctl_file * ctl)
>  	spin_unlock_irqrestore(&ctl->read_lock, flags);
>  }
>  
> +static int snd_ctl_flush(struct file *filp, fl_owner_t id)
> +{
> +	struct snd_ctl_file *cfile = filp->private_data;
> +
> +	spin_lock(&cfile->read_lock);
> +
> +	/* Release blocking operations. */
> +	cfile->subscribed = 0;
> +	wake_up(&cfile->change_sleep);
> +
> +	spin_unlock(&cfile->read_lock);
> +
> +	return 0;
> +}
> +
>  static int snd_ctl_release(struct inode *inode, struct file *file)
>  {
>  	unsigned long flags;
> @@ -1380,13 +1395,16 @@ static int snd_ctl_subscribe_events(struct snd_ctl_file *file, int __user *ptr)
>  			return -EFAULT;
>  		return 0;
>  	}
> +
> +	/* TODO: mutual exclusives. */
>  	if (subscribe) {
>  		file->subscribed = 1;
> -		return 0;
>  	} else if (file->subscribed) {
>  		snd_ctl_empty_read_queue(file);
>  		file->subscribed = 0;
>  	}
> +	wake_up(&cfile->change_sleep);
> +
>  	return 0;
>  }
>  
> @@ -1553,6 +1571,8 @@ static ssize_t snd_ctl_read(struct file *file, char __user *buffer,
>  			remove_wait_queue(&ctl->change_sleep, &wait);
>  			if (ctl->card->shutdown)
>  				return -ENODEV;
> +			if (ctl->subscribed == 0)
> +				return -EBADFD;
>  			if (signal_pending(current))
>  				return -ERESTARTSYS;
>  			spin_lock_irq(&ctl->read_lock);
> @@ -1581,17 +1601,20 @@ static ssize_t snd_ctl_read(struct file *file, char __user *buffer,
>  
>  static unsigned int snd_ctl_poll(struct file *file, poll_table * wait)
>  {
> -	unsigned int mask;
> -	struct snd_ctl_file *ctl;
> +	struct snd_ctl_file *ctl = file->private_data;
> +	unsigned int mask = 0;
>  
> -	ctl = file->private_data;
>  	if (!ctl->subscribed)
>  		return 0;
> +
>  	poll_wait(file, &ctl->change_sleep, wait);
>  
> -	mask = 0;
> -	if (!list_empty(&ctl->events))
> -		mask |= POLLIN | POLLRDNORM;
> +	spin_lock(&ctl->read_lock);
> +
> +	if (!list_empty(&ctl->events) && ctl->subscribed == 1)
> +		mask = POLLIN | POLLRDNORM;
> +
> +	spin_unlock(&ctl->read_lock);
>  
>  	return mask;
>  }
> @@ -1733,6 +1756,7 @@ static const struct file_operations snd_ctl_f_ops =
>  	.owner =	THIS_MODULE,
>  	.read =		snd_ctl_read,
>  	.open =		snd_ctl_open,
> +	.flush =	snd_ctl_flush,
>  	.release =	snd_ctl_release,
>  	.llseek =	no_llseek,
>  	.poll =		snd_ctl_poll,
> -- 
> 2.1.4
>