[RFC PATCH v2 2/2] ALSA: pcm: refresh capture pointers before drain

[Joonho Lee <mynameisjune111@gmail.com>]

Refresh active capture pointers before the drain transition so the final
captured tail is accounted before reads finish.

Keep the link rwsem held through pointer refresh and the drain action.
Release the initiating stream lock while refreshing, lock each running
capture member only while updating its pointer, then reacquire the
initiating lock before the state transition. Pair the rwsem on every
exit and retain the existing error cleanup path.

Assisted-by: LLM
Signed-off-by: Joonho Lee <mynameisjune111@gmail.com>
---
 sound/core/pcm_native.c | 26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index defbb2977efe..5375f8f931d9 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -2176,14 +2176,32 @@ static int snd_pcm_drain(struct snd_pcm_substream *substream,
 	} else if (substream->f_flags & O_NONBLOCK)
 		nonblock = 1;
 
+	/* Keep linked runtimes alive until the initial drain action completes. */
+	down_read(&snd_pcm_link_rwsem);
 	snd_pcm_stream_lock_irq(substream);
-	/* resume pause */
+	/* Resume before refreshing capture positions. */
 	if (runtime->state == SNDRV_PCM_STATE_PAUSED)
 		snd_pcm_pause(substream, false);
+	snd_pcm_stream_unlock_irq(substream);
+
+	/* A pointer error may stop the group: hold only this member's lock. */
+	snd_pcm_group_for_each_entry(s, substream) {
+		if (s->stream != SNDRV_PCM_STREAM_CAPTURE)
+			continue;
+		snd_pcm_stream_lock_irq(s);
+		if (s->runtime->state == SNDRV_PCM_STATE_RUNNING)
+			result = snd_pcm_update_hw_ptr(s);
+		snd_pcm_stream_unlock_irq(s);
+		if (result < 0)
+			break;
+	}
 
-	/* pre-start/stop - all running streams are changed to DRAINING state */
-	result = snd_pcm_action(&snd_pcm_action_drain_init, substream,
-				ACTION_ARG_IGNORE);
+	snd_pcm_stream_lock_irq(substream);
+	/* Change the stable group to DRAINING only after all refreshes. */
+	if (result >= 0)
+		result = snd_pcm_action(&snd_pcm_action_drain_init,
+					substream, ACTION_ARG_IGNORE);
+	up_read(&snd_pcm_link_rwsem);
 	if (result < 0)
 		goto unlock;
 	/* in non-blocking, we don't wait in ioctl but let caller poll */