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

[Takashi Iwai <tiwai@suse.de>]

On Wed, 23 Mar 2022 09:08:25 +0100,
Amadeusz SX2awiX4ski wrote:
> 
> 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);
> ?

No, it must be nested locks with the given subclass.  That's why it
has been the open code beforehand, too.

> > @@ -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?

This side would be possible to use that macro but it's still better to
have the consistent call pattern.


thanks,

Takashi