Re: [alsa-devel] Clean up of PCM spinlocks

Date 13.3.2012 11:57, Takashi Iwai wrote:

Hi,

below is a patch I posted to LKML as the follow up for the lockdep report from Dave Jones. It's a resurrected patch from what I worked on quite ago. Although this fix is suboptimal, I see no better way.

If this looks OK, I'm going to queue it as a 3.4 material.

thanks,

Takashi

===

From: Takashi Iwai tiwai@suse.de Subject: [PATCH] ALSA: pcm - Simplify linked PCM substream spinlocks

The spinlocks held for PCM substreams have been always messy. For the better concurrent accessibility, we took always the substream's lock first. When substreams are linked, we need a group-wide lock, but this should be applied outside substream's lock. So, we unlock the substream's lock first, then do locks twice.

This scheme is known to be problematic with lockdep. Maybe because the nested lock isn't marked properly, and partly because the lock and unlock sequences are different (A/B -> A/B).

This patch tries to simplify the scheme. Instead of holding the substream's lock, we take the group lock always at first. A drawback of this is that the access to the individual substream in a same group won't be allowed any longer. But, the code looks much easier, and likely less buggy.

What do you mean by this? I noticed that you switched the behaviour for the stream_lock():

1) self_group spinlock is used for single (non-linked) stream - same as before 2) group.lock spinlock is used for the linked streams - the self_group spinlock is used only in the group action - the question is why? it seems useless, the stream_lock() functions use the allocated group.lock

I don't think that this patch will work in the way we expect.

I believe we should use only group.lock spinlock for the linked streams without using the self_group spinlock to make locking work like this:

stream_lock(current) group_action(current) { for s from all_group_substreams { if (s != current) stream_lock(s); } .... job .... .... group substreams unlock .... } stream_unlock(current)

So we use only one lock per stream.

Jaroslav

Note that the group lock is identical with the substream's lock initially before the substream is linked with others. So, in the case of non-linked PCM, the new code behaves exactly same as before.

Signed-off-by: Takashi Iwai tiwai@suse.de

include/sound/pcm.h | 12 ++++++------ sound/core/pcm_native.c | 36 ++++++++++++++---------------------- 2 files changed, 20 insertions(+), 28 deletions(-)

diff --git a/include/sound/pcm.h b/include/sound/pcm.h index 0cf91b2..d634dcc 100644 --- a/include/sound/pcm.h +++ b/include/sound/pcm.h @@ -531,36 +531,36 @@ static inline int snd_pcm_stream_linked(struct snd_pcm_substream *substream) static inline void snd_pcm_stream_lock(struct snd_pcm_substream *substream) { read_lock(&snd_pcm_link_rwlock);

• spin_lock(&substream->self_group.lock);

• spin_lock(&substream->group->lock);

}

static inline void snd_pcm_stream_unlock(struct snd_pcm_substream *substream) {

• spin_unlock(&substream->self_group.lock);

• spin_unlock(&substream->group->lock); read_unlock(&snd_pcm_link_rwlock);

}

static inline void snd_pcm_stream_lock_irq(struct snd_pcm_substream *substream) { read_lock_irq(&snd_pcm_link_rwlock);

• spin_lock(&substream->self_group.lock);

• spin_lock(&substream->group->lock);

}

static inline void snd_pcm_stream_unlock_irq(struct snd_pcm_substream *substream) {

• spin_unlock(&substream->self_group.lock);

• spin_unlock(&substream->group->lock); read_unlock_irq(&snd_pcm_link_rwlock);

}

#define snd_pcm_stream_lock_irqsave(substream, flags) \ do { \ read_lock_irqsave(&snd_pcm_link_rwlock, (flags)); \

• spin_lock(&substream->self_group.lock); \

• spin_lock(&substream->group->lock); \

} while (0)

#define snd_pcm_stream_unlock_irqrestore(substream, flags) \ do { \

• spin_unlock(&substream->self_group.lock); \

• spin_unlock(&substream->group->lock); \ read_unlock_irqrestore(&snd_pcm_link_rwlock, (flags)); \

} while (0)

diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c index 25ed9fe..c5f28ed 100644 --- a/sound/core/pcm_native.c +++ b/sound/core/pcm_native.c @@ -712,7 +712,7 @@ static int snd_pcm_action_group(struct action_ops *ops, int res = 0;

snd_pcm_group_for_each_entry(s, substream) {

• if (do_lock && s != substream)
  

• if (do_lock)
  spin_lock_nested(&s->self_group.lock,
  		 SINGLE_DEPTH_NESTING);
  

  res = ops->pre_action(s, state);

@@ -740,8 +740,7 @@ static int snd_pcm_action_group(struct action_ops *ops, if (do_lock) { /* unlock streams */ snd_pcm_group_for_each_entry(s1, substream) {

• 	if (s1 != substream)
  

• 		spin_unlock(&s1->self_group.lock);
  

• 	spin_unlock(&s1->self_group.lock);
  if (s1 == s)	/* end */
  	break;
  

  }

@@ -779,13 +778,7 @@ static int snd_pcm_action(struct action_ops *ops, int res;

if (snd_pcm_stream_linked(substream)) {

• if (!spin_trylock(&substream->group->lock)) {
  

• 	spin_unlock(&substream->self_group.lock);
  

• 	spin_lock(&substream->group->lock);
  

• 	spin_lock(&substream->self_group.lock);
  

• }
  

  res = snd_pcm_action_group(ops, substream, state, 1);

• spin_unlock(&substream->group->lock);
  

  } else { res = snd_pcm_action_single(ops, substream, state); }

@@ -801,19 +794,13 @@ static int snd_pcm_action_lock_irq(struct action_ops *ops, { int res;

• read_lock_irq(&snd_pcm_link_rwlock);

• snd_pcm_stream_lock_irq(substream); if (snd_pcm_stream_linked(substream)) {

• spin_lock(&substream->group->lock);
  

• spin_lock(&substream->self_group.lock);
  

  res = snd_pcm_action_group(ops, substream, state, 1);

• spin_unlock(&substream->self_group.lock);
  

• spin_unlock(&substream->group->lock);
  

  } else {

• spin_lock(&substream->self_group.lock);
  

  res = snd_pcm_action_single(ops, substream, state);

• spin_unlock(&substream->self_group.lock);
  

  }
• read_unlock_irq(&snd_pcm_link_rwlock);

• snd_pcm_stream_unlock_irq(substream); return res;

}

@@ -1586,12 +1573,18 @@ static int snd_pcm_link(struct snd_pcm_substream *substream, int fd) struct file *file; struct snd_pcm_file *pcm_file; struct snd_pcm_substream *substream1;

• struct snd_pcm_group *group;

  file = snd_pcm_file_fd(fd); if (!file) return -EBADFD; pcm_file = file->private_data; substream1 = pcm_file->substream;

• group = kmalloc(sizeof(*group), GFP_KERNEL);

• if (!group) {

• res = -ENOMEM;
  

• goto _nolock;
  

• } down_write(&snd_pcm_link_rwsem); write_lock_irq(&snd_pcm_link_rwlock); if (substream->runtime->status->state == SNDRV_PCM_STATE_OPEN ||

@@ -1604,11 +1597,7 @@ static int snd_pcm_link(struct snd_pcm_substream *substream, int fd) goto _end; } if (!snd_pcm_stream_linked(substream)) {

• substream->group = kmalloc(sizeof(struct snd_pcm_group), GFP_ATOMIC);
  

• if (substream->group == NULL) {
  

• 	res = -ENOMEM;
  

• 	goto _end;
  

• }
  

• substream->group = group;
  

  spin_lock_init(&substream->group->lock); INIT_LIST_HEAD(&substream->group->substreams); list_add_tail(&substream->link_list, &substream->group->substreams);

@@ -1620,7 +1609,10 @@ static int snd_pcm_link(struct snd_pcm_substream *substream, int fd) _end: write_unlock_irq(&snd_pcm_link_rwlock); up_write(&snd_pcm_link_rwsem);

• _nolock: fput(file);
• if (res < 0)

• kfree(group);
  

  return res;

}