Re: [PATCH 4.4 20/87] ALSA: pcm: Allow aborting mutex lock at OSS read/write loops

[Takashi Iwai]

On Wed, 24 Jan 2018 00:35:48 +0100,
Ben Hutchings wrote:
> 
> On Mon, 2018-01-15 at 13:34 +0100, Greg Kroah-Hartman wrote:
> > 4.4-stable review patch.ÂÂIf anyone has any objections, please let me know.
> > 
> > ------------------
> > 
> > From: Takashi Iwai <tiwai@xxxxxxx>
> > 
> > commit 900498a34a3ac9c611e9b425094c8106bdd7dc1c upstream.
> > 
> > PCM OSS read/write loops keep taking the mutex lock for the whole
> > read/write, and this might take very long when the exceptionally high
> > amount of data is given.ÂÂAlso, since it invokes with mutex_lock(),
> > the concurrent read/write becomes unbreakable.
> > 
> > This patch tries to address these issues by replacing mutex_lock()
> > with mutex_lock_interruptible(), and also splits / re-takes the lock
> > at each read/write period chunk, so that it can switch the context
> > more finely if requested.
> [...]
> > @@ -1414,18 +1417,18 @@ static ssize_t snd_pcm_oss_write1(struct
> > Â			xfer += tmp;
> > Â			if ((substream->f_flags & O_NONBLOCK) != 0 &&
> > Â			ÂÂÂÂtmp != runtime->oss.period_bytes)
> > -				break;
> > +				tmp = -EAGAIN;
> > Â		}
> > + err:
> > +		mutex_unlock(&runtime->oss.params_lock);
> > +		if (tmp < 0)
> > +			break;
> > Â		if (signal_pending(current)) {
> > Â			tmp = -ERESTARTSYS;
> > -			goto err;
> > +			break;
> > Â		}
> > +		tmp = 0;
> > Â	}
> > -	mutex_unlock(&runtime->oss.params_lock);
> > -	return xfer;
> > -
> > - err:
> > -	mutex_unlock(&runtime->oss.params_lock);
> > Â	return xfer > 0 ? (snd_pcm_sframes_t)xfer : tmp;
> > Â}
> [...]
> 
> Some of the "goto err" statements in the loop are conditional on tmp <=
> 0, but if tmp == 0 this will no longer terminate the loop.  Is that
> intentional or a bug?

The patch rather fixes the endless loop: the signal_pending() check is
added after goto err, so that it aborts the loop properly.


thanks,

Takashi