ALSA: pcm: Fix refcount_inc() on zero usage

The recent rewrite of PCM link lock management introduced the refcount
in snd_pcm_group object, managed by the kernel refcount_t API.  This
caused unexpected kernel warnings when the kernel is built with
CONFIG_REFCOUNT_FULL=y.  As the warning line indicates, the problem is
obviously that we start with refcount=0 and do refcount_inc() for
adding each PCM link, while refcount_t API doesn't like refcount_inc()
performed on zero.

For adapting the proper refcount_t usage, this patch changes the logic
slightly:
- The initial refcount is 1, assuming the single list entry
- The refcount is incremented / decremented at each PCM link addition
  and deletion
- ... which allows us concentrating only on the refcount as a release
  condition

Fixes: f57f3df03a8e ("ALSA: pcm: More fine-grained PCM link locking")
BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=204221
Reported-and-tested-by: Duncan Overbruck <kernel@duncano.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index 860543a4c840207cda8e4a8c3ec873868b27b8ed..12dd9b318db18adcc087c64ae5a4db7e9661d30f 100644 (file)
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -77,7 +77,7 @@ void snd_pcm_group_init(struct snd_pcm_group *group)
        spin_lock_init(&group->lock);
        mutex_init(&group->mutex);
        INIT_LIST_HEAD(&group->substreams);
-       refcount_set(&group->refs, 0);
+       refcount_set(&group->refs, 1);
 }
 
 /* define group lock helpers */
@@ -1096,8 +1096,7 @@ static void snd_pcm_group_unref(struct snd_pcm_group *group,
 
        if (!group)
                return;
-       do_free = refcount_dec_and_test(&group->refs) &&
-               list_empty(&group->substreams);
+       do_free = refcount_dec_and_test(&group->refs);
        snd_pcm_group_unlock(group, substream->pcm->nonatomic);
        if (do_free)
                kfree(group);
@@ -2020,6 +2019,7 @@ static int snd_pcm_link(struct snd_pcm_substream *substream, int fd)
        snd_pcm_group_lock_irq(target_group, nonatomic);
        snd_pcm_stream_lock(substream1);
        snd_pcm_group_assign(substream1, target_group);
+       refcount_inc(&target_group->refs);
        snd_pcm_stream_unlock(substream1);
        snd_pcm_group_unlock_irq(target_group, nonatomic);
  _end:
@@ -2056,13 +2056,14 @@ static int snd_pcm_unlink(struct snd_pcm_substream *substream)
        snd_pcm_group_lock_irq(group, nonatomic);
 
        relink_to_local(substream);
+       refcount_dec(&group->refs);
 
        /* detach the last stream, too */
        if (list_is_singular(&group->substreams)) {
                relink_to_local(list_first_entry(&group->substreams,
                                                 struct snd_pcm_substream,
                                                 link_list));
-               do_free = !refcount_read(&group->refs);
+               do_free = refcount_dec_and_test(&group->refs);
        }
 
        snd_pcm_group_unlock_irq(group, nonatomic);