[alsa-devel] [PATCH] pcm_hw: Respect LIBASOUND_THREAD_SAFE env var

Fri Sep 2 08:55:39 CEST 2016

On Thu, 01 Sep 2016 21:40:50 +0200,
David Henningsson wrote:
> 
> On 2016-09-01 14:28, Takashi Iwai wrote:
> > On Thu, 01 Sep 2016 08:51:14 +0200,
> > Takashi Iwai wrote:
> >>
> >> On Thu, 01 Sep 2016 08:06:50 +0200,
> >> David Henningsson wrote:
> >>>
> >>> Without this patch, there no way to revert to the old
> >>> behaviour with pcm_hw, so currently there will always
> >>> be some extra locking.
> >>>
> >>> This seems to be an oversight?
> >>
> >> thread_safe=1 means that the plugin needs no locking, so it means hw
> >> PCM never takes a lock by itself.  Only other plugins do, and it's
> >> suppressed when $LIBASOUND_THREAD_SAFE=0 is passed.
> >>
> >> The flag needs a better description or a better name...
> >
> > So here we go, let's decrypt the monkey code.
> > Let me know if this is good enough.
> 
> This is better, indeed. It also fixes the bug I sent a patch for
> (Respect LIBASOUND_THREAD_SAFE env var).

The behavior doesn't change.  The old code treated the flag as
tristate while now having two booleans instead.  That's the only
difference.

> It's perhaps a bit inconsistent that "true" is -1 for lock_enabled and
> 1 for need_lock, but that's just a nitpick.

No, -1 is set just to indicate the uninitialized state.  At the first
call, it's replaced with either 0 or 1.

> Since many calls to __snd_pcm_lock has the comment "forced lock"
> perhaps it can be renamed to snd_pcm_lock_forced, and snd_pcm_lock can
> perhaps be snd_pcm_lock_if_needed?

I don't think it matters much.  They are internal function calls,
after all.  And the __ variant shouldn't be called blindly, only with
care.

Takashi

> >
> >
> > Takashi
> >
> > -- 8< --
> > From: Takashi Iwai <tiwai at suse.de>
> > Subject: [PATCH] pcm: Better understandable locking code
> >
> > The newly added locking code seems to have confused quite a few
> > people, as "thread_safe=1" may be considered as if the thread-safety
> > lock has to be turned on.  (It meant that the plugin _is_ thread-safe,
> > i.e. it needs no extra locking.)
> >
> > For avoiding such a misunderstanding, this commit renames the relevant
> > pcm fields and give more comments to explain what is for what.
> > The former single pcm->thread_safe flag is now split to two boolean
> > flags, pcm->need_lock and pcm->lock_enabled.  It consumes a few more
> > bytes, but this would be (hopefully) better understandable.
> >
> > No functional change by this commit.
> >
> > Signed-off-by: Takashi Iwai <tiwai at suse.de>
> > ---
> >  src/pcm/pcm.c       | 16 +++++++++++-----
> >  src/pcm/pcm_hw.c    |  2 +-
> >  src/pcm/pcm_local.h | 27 ++++++++++++++++++++++-----
> >  3 files changed, 34 insertions(+), 11 deletions(-)
> >
> > diff --git a/src/pcm/pcm.c b/src/pcm/pcm.c
> > index f8323999343e..c96ae1c4641a 100644
> > --- a/src/pcm/pcm.c
> > +++ b/src/pcm/pcm.c
> > @@ -2545,14 +2545,20 @@ int snd_pcm_new(snd_pcm_t **pcmp, snd_pcm_type_t type, const char *name,
> >  	INIT_LIST_HEAD(&pcm->async_handlers);
> >  #ifdef THREAD_SAFE_API
> >  	pthread_mutex_init(&pcm->lock, NULL);
> > +	/* use locking as default;
> > +	 * each plugin may suppress this in its open call
> > +	 */
> > +	pcm->need_lock = 1;
> >  	{
> > -		static int default_thread_safe = -1;
> > -		if (default_thread_safe < 0) {
> > +		/* set lock_enabled field depending on $LIBASOUND_THREAD_SAFE;
> > +		 * it's evaluated only once at the first open for consistency
> > +		 */
> > +		static int do_lock_enable = -1;
> > +		if (do_lock_enable < 0) {
> >  			char *p = getenv("LIBASOUND_THREAD_SAFE");
> > -			default_thread_safe = !p || *p != '0';
> > +			do_lock_enable = !p || *p != '0';
> >  		}
> > -		if (!default_thread_safe)
> > -			pcm->thread_safe = -1; /* force to disable */
> > +		pcm->lock_enabled = do_lock_enable;
> >  	}
> >  #endif
> >  	*pcmp = pcm;
> > diff --git a/src/pcm/pcm_hw.c b/src/pcm/pcm_hw.c
> > index 3a5634c1d39a..56e88b6bf6c0 100644
> > --- a/src/pcm/pcm_hw.c
> > +++ b/src/pcm/pcm_hw.c
> > @@ -1514,7 +1514,7 @@ int snd_pcm_hw_open_fd(snd_pcm_t **pcmp, const char *name,
> >  	pcm->poll_events = info.stream == SND_PCM_STREAM_PLAYBACK ? POLLOUT : POLLIN;
> >  	pcm->tstamp_type = tstamp_type;
> >  #ifdef THREAD_SAFE_API
> > -	pcm->thread_safe = 1;
> > +	pcm->need_lock = 0;	/* hw plugin is thread-safe */
> >  #endif
> >
> >  	ret = snd_pcm_hw_mmap_status(pcm);
> > diff --git a/src/pcm/pcm_local.h b/src/pcm/pcm_local.h
> > index bb7964d7833e..bba2f15ac463 100644
> > --- a/src/pcm/pcm_local.h
> > +++ b/src/pcm/pcm_local.h
> > @@ -244,7 +244,12 @@ struct _snd_pcm {
> >  	void *private_data;
> >  	struct list_head async_handlers;
> >  #ifdef THREAD_SAFE_API
> > -	int thread_safe;
> > +	int need_lock;		/* true = this PCM (plugin) is thread-unsafe,
> > +				 * thus it needs a lock.
> > +				 */
> > +	int lock_enabled;	/* thread-safety lock is enabled on the system;
> > +				 * it's set depending on $LIBASOUND_THREAD_SAFE.
> > +				 */
> >  	pthread_mutex_t lock;
> >  #endif
> >  };
> > @@ -1085,24 +1090,36 @@ static inline void sw_set_period_event(snd_pcm_sw_params_t *params, int val)
> >  #define PCMINABORT(pcm) (((pcm)->mode & SND_PCM_ABORT) != 0)
> >
> >  #ifdef THREAD_SAFE_API
> > +/*
> > + * __snd_pcm_lock() and __snd_pcm_unlock() are used to lock/unlock the plugin
> > + * forcibly even if it's declared as thread-safe.  It's needed only for some
> > + * codes that are thread-unsafe per design (e.g. snd_pcm_nonblock()).
> > + *
> > + * OTOH, snd_pcm_lock() and snd_pcm_unlock() are used to lock/unlock the plugin
> > + * in normal situations.  They do lock/unlock only when the plugin is
> > + * thread-unsafe.
> > + *
> > + * Both __snd_pcm_lock() and snd_pcm_lock() (and their unlocks) wouldn't do
> > + * any action when the whole locking is disabled via $LIBASOUND_THREAD_SAFE=0.
> > + */
> >  static inline void __snd_pcm_lock(snd_pcm_t *pcm)
> >  {
> > -	if (pcm->thread_safe >= 0)
> > +	if (pcm->lock_enabled)
> >  		pthread_mutex_lock(&pcm->lock);
> >  }
> >  static inline void __snd_pcm_unlock(snd_pcm_t *pcm)
> >  {
> > -	if (pcm->thread_safe >= 0)
> > +	if (pcm->lock_enabled)
> >  		pthread_mutex_unlock(&pcm->lock);
> >  }
> >  static inline void snd_pcm_lock(snd_pcm_t *pcm)
> >  {
> > -	if (!pcm->thread_safe)
> > +	if (pcm->lock_enabled && pcm->need_lock)
> >  		pthread_mutex_lock(&pcm->lock);
> >  }
> >  static inline void snd_pcm_unlock(snd_pcm_t *pcm)
> >  {
> > -	if (!pcm->thread_safe)
> > +	if (pcm->lock_enabled && pcm->need_lock)
> >  		pthread_mutex_unlock(&pcm->lock);
> >  }
> >  #else /* THREAD_SAFE_API */
> >
> 