From a41c4cb913b53bf74f1ec66a4b96057626c87009 Mon Sep 17 00:00:00 2001 From: Takashi Iwai Date: Sun, 13 Jan 2019 09:40:21 +0100 Subject: ALSA: pcm: Make PCM linked list consistent while re-grouping Make a common helper to re-assign the PCM link using list_move() instead of open code with manual list_del() and list_add_tail(). This assures the consistency and we can get rid of snd_pcm_group.count field -- its purpose is only to check whether the list is singular, and we can know it by list_is_singular() call now. Signed-off-by: Takashi Iwai --- include/sound/pcm.h | 1 - 1 file changed, 1 deletion(-) (limited to 'include/sound') diff --git a/include/sound/pcm.h b/include/sound/pcm.h index d6bd3caf6878..e1c747c70883 100644 --- a/include/sound/pcm.h +++ b/include/sound/pcm.h @@ -439,7 +439,6 @@ struct snd_pcm_group { /* keep linked substreams */ spinlock_t lock; struct mutex mutex; struct list_head substreams; - int count; }; struct pid; -- cgit From f57f3df03a8e6010e321fa0258d3e054713c3cb7 Mon Sep 17 00:00:00 2001 From: Takashi Iwai Date: Sun, 13 Jan 2019 09:50:33 +0100 Subject: ALSA: pcm: More fine-grained PCM link locking We have currently two global locks, a rwlock and a rwsem, that are used for managing linking the PCM streams. Due to these global locks, once when a linked stream is used, the lock granularity suffers a lot. This patch attempts to eliminate the former global lock for atomic ops. The latter rwsem needs remaining because of the loosy way of the loop calls in snd_pcm_action_nonatomic(), as well as for avoiding the deadlock at linking. However, these are used far rarely, actually only by two actions (prepare and reset), where both are no timing critical ones. So this can be still seen as a good improvement. The basic strategy to eliminate the rwlock is to assure group->lock at adding or removing a stream to / from the group. Since we already takes the group lock whenever taking the all substream locks under the group, this shouldn't be a big problem. The reference to group pointer in snd_pcm_substream object is protected by the stream lock itself. However, there are still pitfalls: a race window at re-locking and the lifecycle of group object. The former is a small race window for dereferencing the substream group object opened while snd_pcm_action() performs re-locking to avoid ABBA deadlocks. This includes the unlink of group during that window, too. And the latter is the kfree performed after all streams are removed from the group while it's still dereferenced. For addressing these corner cases, two new tricks are introduced: - After re-locking, the group assigned to the stream is checked again; if the group is changed, we retry the whole procedure. - Introduce a refcount to snd_pcm_group object, so that it's freed only when it's empty and really no one refers to it. (Some readers might wonder why not RCU for the latter. RCU in this case would cost more than refcounting, unfortunately. We take the group lock sooner or later, hence the performance improvement by RCU would be negligible. Meanwhile, because we need to deal with schedulable context depending on the pcm->nonatomic flag, it'll become dynamic RCU/SRCU switch, and the grace period may become too long.) Along with these changes, there are a significant amount of code refactoring. The complex group re-lock & ref code is factored out to snd_pcm_stream_group_ref() function, for example. Signed-off-by: Takashi Iwai --- include/sound/pcm.h | 2 ++ 1 file changed, 2 insertions(+) (limited to 'include/sound') diff --git a/include/sound/pcm.h b/include/sound/pcm.h index e1c747c70883..3bde24575a99 100644 --- a/include/sound/pcm.h +++ b/include/sound/pcm.h @@ -30,6 +30,7 @@ #include #include #include +#include #define snd_pcm_substream_chip(substream) ((substream)->private_data) #define snd_pcm_chip(pcm) ((pcm)->private_data) @@ -439,6 +440,7 @@ struct snd_pcm_group { /* keep linked substreams */ spinlock_t lock; struct mutex mutex; struct list_head substreams; + refcount_t refs; }; struct pid; -- cgit