On Mon, 19 Jun 2017 14:10:00 +0200, Takashi Sakamoto wrote:
On Jun 19 2017 02:23, Takashi Iwai wrote:
I'll post a patch to take 'snd_pcm_reset()' to use 'snd_pcm_action_lock_irq()' instead of 'snd_pcm_action_nonatomic()'. How do you think about it?
Conceptually seen, that's not right. snd_pcm_reset() calls PCM ioctl ops, and this is supposed to be always non-atomic (it's a kind of ioctl wrapper, after all). So the correct fix would be simply to wrap only the snd_pcm_playback_silence() call with the stream lock.
I once investigated for this idea. But I was discouraged because of several resons:
- There's a semantic contradiction. 'snd_pcm_action_nonatomic()'
includes atomic operation. The idea perhaps puzzles developers.
Oh no, that's a big misunderstanding. snd_pcm_action_nonatomic() doesn't mean to prohibit the critical section protected via a spinlock inside its execution code. Instead, it's the code you can't call *from* spinlocked context.
- In 'snd_pcm_pre_reset()', 'snd_pcm_do_reset()' and
'snd_pcm_post_reset()', parameters of runtime for PCM substream are also changed or referred. They can be changed in other functions concurrently, thus should be locked.
They are protected in terms of the mutex.
- Currently there're a few drivers which implements the local
ioctl. All of them handle the callback without calling task scheduler. It's reasonable against the cost to have an additional restraint just for context with the reset command.
But the local ioctl *is* always non-atomic, that's the reason I wrote "conceptually seen". If you don't want it to be non-atomic, you'd need to introduce yet another ops for doing that.
Well, this is a revised version.
diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c index 7e8f3656b695..c74bee099155 100644 --- a/sound/core/pcm_native.c +++ b/sound/core/pcm_native.c @@ -1593,7 +1593,11 @@ static int snd_pcm_pre_reset(struct snd_pcm_substream *substream, int state) static int snd_pcm_do_reset(struct snd_pcm_substream *substream, int state) { struct snd_pcm_runtime *runtime = substream->runtime;
int err = substream->ops->ioctl(substream,SNDRV_PCM_IOCTL1_RESET, NULL);
int err;snd_pcm_stream_unlock_irq(substream);err = substream->ops->ioctl(substream, SNDRV_PCM_IOCTL1_RESET,NULL);
snd_pcm_stream_lock_irq(substream); if (err < 0) return err; runtime->hw_ptr_base = 0;@@ -1621,7 +1625,7 @@ static const struct action_ops snd_pcm_action_reset = {
static int snd_pcm_reset(struct snd_pcm_substream *substream) {
return snd_pcm_action_nonatomic(&snd_pcm_action_reset,substream, 0);
return snd_pcm_action_lock_irq(&snd_pcm_action_reset, substream, 0);}
/*
There's still a contradiction about atomicity but as a whole better than before.
No, this code is just wrong... The PCM locking doesn't work like that.
Imagine multiple PCM streams are linked. Then you'll be screwed up.
thanks,
Takashi