[alsa-devel] [PATCH 1/2] ALSA: seq: virmidi: Offload the output event processing

[Takashi Iwai]

The virmidi sequencer stuff tries to translate the rawmidi bytes to
sequencer events and deliver the packets at trigger callback.  The
amount of the whole process of these translations and deliveries
depends on the incoming rawmidi bytes, and we have no limit for that;
this was the cause of a CPU soft lockup that had been reported and
fixed recently.

Although we've fixed the soft lockup by putting the temporary unlock
and cond_resched(), it's rather a quick band aid.  In this patch,
meanwhile, the event parsing and delivery process is offloaded to a
dedicated work, and the trigger callback just kicks it off.  It has
three merits, at least:

- The processing is always done in a sleepable context, which can
  assure the event delivery with non-atomic flag without hackish
  is_atomic() usage.

- Other relevant codes can be simplified, reducing the lines

- It makes me happier

Signed-off-by: Takashi Iwai <tiwai at suse.de>
---
 include/sound/seq_virmidi.h  |   1 +
 sound/core/seq/seq_virmidi.c | 100 ++++++++++++++++-------------------
 2 files changed, 47 insertions(+), 54 deletions(-)

diff --git a/include/sound/seq_virmidi.h b/include/sound/seq_virmidi.h
index 695257ae64ac..d488dcfa3a4e 100644
--- a/include/sound/seq_virmidi.h
+++ b/include/sound/seq_virmidi.h
@@ -41,6 +41,7 @@ struct snd_virmidi {
 	struct snd_seq_event event;
 	struct snd_virmidi_dev *rdev;
 	struct snd_rawmidi_substream *substream;
+	struct work_struct output_work;
 };
 
 #define SNDRV_VIRMIDI_SUBSCRIBE		(1<<0)
diff --git a/sound/core/seq/seq_virmidi.c b/sound/core/seq/seq_virmidi.c
index 8ebbca554e99..67ea5d62cebc 100644
--- a/sound/core/seq/seq_virmidi.c
+++ b/sound/core/seq/seq_virmidi.c
@@ -154,70 +154,58 @@ static void snd_virmidi_input_trigger(struct snd_rawmidi_substream *substream, i
 	}
 }
 
-/*
- * trigger rawmidi stream for output
+/* process rawmidi bytes and send events;
+ * we need no lock here for vmidi->event since it's handled only in this work
  */
-static void snd_virmidi_output_trigger(struct snd_rawmidi_substream *substream, int up)
+static void snd_vmidi_output_work(struct work_struct *work)
 {
-	struct snd_virmidi *vmidi = substream->runtime->private_data;
-	int count, res;
-	unsigned char buf[32], *pbuf;
-	unsigned long flags;
-	bool check_resched = !in_atomic();
+	struct snd_virmidi *vmidi;
+	struct snd_rawmidi_substream *substream;
+	unsigned char input;
+	int ret;
+
+	vmidi = container_of(work, struct snd_virmidi, output_work);
+	substream = vmidi->substream;
+
+	/* discard the outputs in dispatch mode unless subscribed */
+	if (vmidi->seq_mode == SNDRV_VIRMIDI_SEQ_DISPATCH &&
+	    !(vmidi->rdev->flags & SNDRV_VIRMIDI_SUBSCRIBE)) {
+		while (!snd_rawmidi_transmit_empty(substream))
+			snd_rawmidi_transmit_ack(substream, 1);
+		return;
+	}
 
-	if (up) {
-		vmidi->trigger = 1;
-		if (vmidi->seq_mode == SNDRV_VIRMIDI_SEQ_DISPATCH &&
-		    !(vmidi->rdev->flags & SNDRV_VIRMIDI_SUBSCRIBE)) {
-			while (snd_rawmidi_transmit(substream, buf,
-						    sizeof(buf)) > 0) {
-				/* ignored */
-			}
-			return;
-		}
-		spin_lock_irqsave(&substream->runtime->lock, flags);
+	while (vmidi->trigger) {
+		if (snd_rawmidi_transmit(substream, &input, 1) != 1)
+			break;
+		if (snd_midi_event_encode_byte(vmidi->parser, input,
+					       &vmidi->event) <= 0)
+			continue;
 		if (vmidi->event.type != SNDRV_SEQ_EVENT_NONE) {
-			if (snd_seq_kernel_client_dispatch(vmidi->client, &vmidi->event, in_atomic(), 0) < 0)
-				goto out;
+			ret = snd_seq_kernel_client_dispatch(vmidi->client,
+							     &vmidi->event,
+							     false, 0);
 			vmidi->event.type = SNDRV_SEQ_EVENT_NONE;
-		}
-		while (1) {
-			count = __snd_rawmidi_transmit_peek(substream, buf, sizeof(buf));
-			if (count <= 0)
+			if (ret < 0)
 				break;
-			pbuf = buf;
-			while (count > 0) {
-				res = snd_midi_event_encode(vmidi->parser, pbuf, count, &vmidi->event);
-				if (res < 0) {
-					snd_midi_event_reset_encode(vmidi->parser);
-					continue;
-				}
-				__snd_rawmidi_transmit_ack(substream, res);
-				pbuf += res;
-				count -= res;
-				if (vmidi->event.type != SNDRV_SEQ_EVENT_NONE) {
-					if (snd_seq_kernel_client_dispatch(vmidi->client, &vmidi->event, in_atomic(), 0) < 0)
-						goto out;
-					vmidi->event.type = SNDRV_SEQ_EVENT_NONE;
-				}
-			}
-			if (!check_resched)
-				continue;
-			/* do temporary unlock & cond_resched() for avoiding
-			 * CPU soft lockup, which may happen via a write from
-			 * a huge rawmidi buffer
-			 */
-			spin_unlock_irqrestore(&substream->runtime->lock, flags);
-			cond_resched();
-			spin_lock_irqsave(&substream->runtime->lock, flags);
 		}
-	out:
-		spin_unlock_irqrestore(&substream->runtime->lock, flags);
-	} else {
-		vmidi->trigger = 0;
+		/* rawmidi input might be huge, allow to have a break */
+		cond_resched();
 	}
 }
 
+/*
+ * trigger rawmidi stream for output
+ */
+static void snd_virmidi_output_trigger(struct snd_rawmidi_substream *substream, int up)
+{
+	struct snd_virmidi *vmidi = substream->runtime->private_data;
+
+	vmidi->trigger = !!up;
+	if (up)
+		queue_work(system_highpri_wq, &vmidi->output_work);
+}
+
 /*
  * open rawmidi handle for input
  */
@@ -270,6 +258,7 @@ static int snd_virmidi_output_open(struct snd_rawmidi_substream *substream)
 	vmidi->port = rdev->port;
 	snd_virmidi_init_event(vmidi, &vmidi->event);
 	vmidi->rdev = rdev;
+	INIT_WORK(&vmidi->output_work, snd_vmidi_output_work);
 	runtime->private_data = vmidi;
 	return 0;
 }
@@ -299,6 +288,9 @@ static int snd_virmidi_input_close(struct snd_rawmidi_substream *substream)
 static int snd_virmidi_output_close(struct snd_rawmidi_substream *substream)
 {
 	struct snd_virmidi *vmidi = substream->runtime->private_data;
+
+	vmidi->trigger = 0; /* to be sure */
+	cancel_work_sync(&vmidi->output_work);
 	snd_midi_event_free(vmidi->parser);
 	substream->runtime->private_data = NULL;
 	kfree(vmidi);
-- 
2.18.0