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

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Date:   Tue, 23 Jan 2018 23:35:48 +0000
From:   Ben Hutchings <ben.hutchings@...ethink.co.uk>
To:     Takashi Iwai <tiwai@...e.de>
Cc:     stable@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4.4 20/87] ALSA: pcm: Allow aborting mutex lock at OSS
 read/write loops

On Mon, 2018-01-15 at 13:34 +0100, Greg Kroah-Hartman wrote:
> 4.4-stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Takashi Iwai <tiwai@...e.de>
> 
> 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.  Also, 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
>  			xfer += tmp;
>  			if ((substream->f_flags & O_NONBLOCK) != 0 &&
>  			    tmp != runtime->oss.period_bytes)
> -				break;
> +				tmp = -EAGAIN;
>  		}
> + err:
> +		mutex_unlock(&runtime->oss.params_lock);
> +		if (tmp < 0)
> +			break;
>  		if (signal_pending(current)) {
>  			tmp = -ERESTARTSYS;
> -			goto err;
> +			break;
>  		}
> +		tmp = 0;
>  	}
> -	mutex_unlock(&runtime->oss.params_lock);
> -	return xfer;
> -
> - err:
> -	mutex_unlock(&runtime->oss.params_lock);
>  	return xfer > 0 ? (snd_pcm_sframes_t)xfer : tmp;
>  }
[...]

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 same for snd_pcm_oss_read1().

Ben.

-- 
Ben Hutchings
Software Developer, Codethink Ltd.