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@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@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.*/#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 */