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 09:15:19 +0100,
Takashi Iwai wrote:
> 
> 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.

FWIW, the reason is that lockdep would complain otherwise as if it
were a deadlock.  That is, this is a workaround for avoiding false
lockdep warnings.


Takashi