[PATCH] ALSA: seq: midi: Optimize event_input locking with RCU

[Takashi Iwai]

The recent fix for serializing the output teardown introduced a
spinlock invocation at every MIDI output event via event_process_midi.
Since this is a hot path, let's do performance optimization with RCU.

The new output_substream __rcu pointer is published via
rcu_assign_pointer() in midisynth_use() after output_rfile is set, and
cleared in midisynth_unuse() before the resource teardown.
event_process_midi() reads it under rcu_read_lock() and bumps
output_use_lock inside that section, which is necessary to close the
window between the pointer dereference and the refcount increment.

midisynth_unuse() calls synchronize_rcu() before snd_use_lock_sync():
this guarantees that any reader who obtained a non-NULL pointer has
already called atomic_inc (output_use_lock), so the subsequent
snd_use_lock_sync() sees the correct in-flight count.

Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
---
 sound/core/seq/seq_midi.c | 24 +++++++++++-------------
 1 file changed, 11 insertions(+), 13 deletions(-)

diff --git a/sound/core/seq/seq_midi.c b/sound/core/seq/seq_midi.c
index 2eb12199c92f..c2f89aee1914 100644
--- a/sound/core/seq/seq_midi.c
+++ b/sound/core/seq/seq_midi.c
@@ -43,8 +43,8 @@ struct seq_midisynth {
 	int device;
 	int subdevice;
 	struct snd_rawmidi_file input_rfile;
-	spinlock_t output_lock;		/* protects output_rfile publication */
 	snd_use_lock_t output_use_lock;	/* in-flight event_input users */
+	struct snd_rawmidi_substream __rcu *output_substream;
 	struct snd_rawmidi_file output_rfile;
 	int seq_client;
 	int seq_port;
@@ -134,8 +134,8 @@ static int event_process_midi(struct snd_seq_event *ev, int direct,
 	if (snd_BUG_ON(!msynth))
 		return -EINVAL;
 
-	scoped_guard(spinlock_irqsave, &msynth->output_lock) {
-		substream = msynth->output_rfile.output;
+	scoped_guard(rcu) {
+		substream = rcu_dereference(msynth->output_substream);
 		if (!substream)
 			return -ENODEV;
 		snd_use_lock_use(&msynth->output_use_lock);
@@ -177,7 +177,6 @@ static int snd_seq_midisynth_new(struct seq_midisynth *msynth,
 	msynth->card = card;
 	msynth->device = device;
 	msynth->subdevice = subdevice;
-	spin_lock_init(&msynth->output_lock);
 	snd_use_lock_init(&msynth->output_use_lock);
 	return 0;
 }
@@ -252,8 +251,8 @@ static int midisynth_use(void *private_data, struct snd_seq_port_subscribe *info
 		return err;
 	}
 	snd_midi_event_reset_decode(msynth->parser);
-	scoped_guard(spinlock_irqsave, &msynth->output_lock)
-		msynth->output_rfile = rfile;
+	msynth->output_rfile = rfile;
+	rcu_assign_pointer(msynth->output_substream, rfile.output);
 	return 0;
 }
 
@@ -261,17 +260,16 @@ static int midisynth_use(void *private_data, struct snd_seq_port_subscribe *info
 static int midisynth_unuse(void *private_data, struct snd_seq_port_subscribe *info)
 {
 	struct seq_midisynth *msynth = private_data;
-	struct snd_rawmidi_file rfile = {};
+	struct snd_rawmidi_file rfile;
 
-	scoped_guard(spinlock_irqsave, &msynth->output_lock) {
-		rfile = msynth->output_rfile;
-		msynth->output_rfile = (struct snd_rawmidi_file){};
-	}
+	rcu_assign_pointer(msynth->output_substream, NULL);
+	synchronize_rcu();
+	snd_use_lock_sync(&msynth->output_use_lock);
+	rfile = msynth->output_rfile;
+	msynth->output_rfile = (struct snd_rawmidi_file){};
 
 	if (snd_BUG_ON(!rfile.output))
 		return -EINVAL;
-
-	snd_use_lock_sync(&msynth->output_use_lock);
 	snd_rawmidi_drain_output(rfile.output);
 	return snd_rawmidi_kernel_release(&rfile);
 }
-- 
2.55.0