Date 13.3.2012 16:00, Takashi Iwai wrote:
At Tue, 13 Mar 2012 15:24:11 +0100, Jaroslav Kysela wrote:
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():
- self_group spinlock is used for single (non-linked) stream
- same as before
- 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
Oh yeah, right, this is utterly useless! I just thought of the mutex case, but in this case, the lock is avoided anyway.
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)
Well, if we think of a single lock, the code would be much simplified than above.
The reviewsed patch is below. I left the single-stream version as is since it's used explicitly in the drain() -- the drain action is applied to each substream although it's linked.
I meant use group->lock in the group_action, see patch bellow.
Also, the changes in snd_pcm_link() to use GFP_KERNEL is splitted out of this patch.
Maybe, we can reuse the self_group container for the linked streams, too? In this case, the dynamic allocation of new group structure can be removed.
Jaroslav
diff --git a/include/sound/pcm.h b/include/sound/pcm.h index 1d58d79..301ed22 100644 --- a/include/sound/pcm.h +++ b/include/sound/pcm.h @@ -535,36 +535,36 @@ 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..5096c83 100644 --- a/sound/core/pcm_native.c +++ b/sound/core/pcm_native.c @@ -713,7 +713,7 @@ static int snd_pcm_action_group(struct action_ops *ops,
snd_pcm_group_for_each_entry(s, substream) { if (do_lock && s != substream) - spin_lock_nested(&s->self_group.lock, + spin_lock_nested(&s->group->lock, SINGLE_DEPTH_NESTING); res = ops->pre_action(s, state); if (res < 0) @@ -741,7 +741,7 @@ static int snd_pcm_action_group(struct action_ops *ops, /* unlock streams */ snd_pcm_group_for_each_entry(s1, substream) { if (s1 != substream) - spin_unlock(&s1->self_group.lock); + spin_unlock(&s1->group->lock); if (s1 == s) /* end */ break; } @@ -779,13 +779,7 @@ 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); + res = snd_pcm_action_group(ops, substream, state); } else { res = snd_pcm_action_single(ops, substream, state); } @@ -801,19 +795,9 @@ static int snd_pcm_action_lock_irq(struct action_ops *ops, { int res;
- read_lock_irq(&snd_pcm_link_rwlock); - 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_lock_irq(substream); + res = snd_pcm_action(ops, substream, state); + snd_pcm_stream_unlock_irq(substream); return res; }