Re: [PATCH 3/4] ALSA: pcm: Fix races among concurrent prepare and hw_params/hw_free calls

["Amadeusz Sławiński" <amadeuszx.slawinski@linux.intel.com>]

On 3/22/2022 6:07 PM, Takashi Iwai wrote:
> Like the previous fixes to hw_params and hw_free ioctl races, we need
> to paper over the concurrent prepare ioctl calls against hw_params and
> hw_free, too.
> 
> This patch implements the locking with the existing
> runtime->buffer_mutex for prepare ioctls.  Unlike the previous case
> for snd_pcm_hw_hw_params() and snd_pcm_hw_free(), snd_pcm_prepare() is
> performed to the linked streams, hence the lock can't be applied
> simply on the top.  For tracking the lock in each linked substream, we
> modify snd_pcm_action_group() slightly and apply the buffer_mutex for
> the case stream_lock=false (formerly there was no lock applied)
> there.
> 
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Takashi Iwai <tiwai@suse.de>
> ---
>   sound/core/pcm_native.c | 32 ++++++++++++++++++--------------
>   1 file changed, 18 insertions(+), 14 deletions(-)
> 
> diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
> index 266895374b83..0e4fbf5fd87b 100644
> --- a/sound/core/pcm_native.c
> +++ b/sound/core/pcm_native.c
> @@ -1190,15 +1190,17 @@ struct action_ops {
>   static int snd_pcm_action_group(const struct action_ops *ops,
>   				struct snd_pcm_substream *substream,
>   				snd_pcm_state_t state,
> -				bool do_lock)
> +				bool stream_lock)
>   {
>   	struct snd_pcm_substream *s = NULL;
>   	struct snd_pcm_substream *s1;
>   	int res = 0, depth = 1;
>   
>   	snd_pcm_group_for_each_entry(s, substream) {
> -		if (do_lock && s != substream) {
> -			if (s->pcm->nonatomic)
> +		if (s != substream) {
> +			if (!stream_lock)
> +				mutex_lock_nested(&s->runtime->buffer_mutex, depth);
> +			else if (s->pcm->nonatomic)
>   				mutex_lock_nested(&s->self_group.mutex, depth);
>   			else
>   				spin_lock_nested(&s->self_group.lock, depth);

Maybe
	if (!stream_lock)
		mutex_lock_nested(&s->runtime->buffer_mutex, depth);
	else
		snd_pcm_group_lock(&s->self_group, s->pcm->nonatomic);
?

> @@ -1226,18 +1228,18 @@ static int snd_pcm_action_group(const struct action_ops *ops,
>   		ops->post_action(s, state);
>   	}
>    _unlock:
> -	if (do_lock) {
> -		/* unlock streams */
> -		snd_pcm_group_for_each_entry(s1, substream) {
> -			if (s1 != substream) {
> -				if (s1->pcm->nonatomic)
> -					mutex_unlock(&s1->self_group.mutex);
> -				else
> -					spin_unlock(&s1->self_group.lock);
> -			}
> -			if (s1 == s)	/* end */
> -				break;
> +	/* unlock streams */
> +	snd_pcm_group_for_each_entry(s1, substream) {
> +		if (s1 != substream) {
> +			if (!stream_lock)
> +				mutex_unlock(&s1->runtime->buffer_mutex);
> +			else if (s1->pcm->nonatomic)
> +				mutex_unlock(&s1->self_group.mutex);
> +			else
> +				spin_unlock(&s1->self_group.lock);

And similarly to above, use snd_pcm_group_unlock() here?

>   		}
> +		if (s1 == s)	/* end */
> +			break;
>   	}
>   	return res;
>   }
> @@ -1367,10 +1369,12 @@ static int snd_pcm_action_nonatomic(const struct action_ops *ops,
>   
>   	/* Guarantee the group members won't change during non-atomic action */
>   	down_read(&snd_pcm_link_rwsem);
> +	mutex_lock(&substream->runtime->buffer_mutex);
>   	if (snd_pcm_stream_linked(substream))
>   		res = snd_pcm_action_group(ops, substream, state, false);
>   	else
>   		res = snd_pcm_action_single(ops, substream, state);
> +	mutex_unlock(&substream->runtime->buffer_mutex);
>   	up_read(&snd_pcm_link_rwsem);
>   	return res;
>   }