[PATCH 3.16 078/328] ALSA: virmidi: Fix too long output trigger loop

[Ben Hutchings]

3.16.62-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Takashi Iwai <tiwai@xxxxxxx>

commit 50e9ffb1996a5d11ff5040a266585bad4ceeca0a upstream.

The virmidi output trigger tries to parse the all available bytes and
process sequencer events as much as possible.  In a normal situation,
this is supposed to be relatively short, but a program may give a huge
buffer and it'll take a long time in a single spin lock, which may
eventually lead to a soft lockup.

This patch simply adds a workaround, a cond_resched() call in the loop
if applicable.  A better solution would be to move the event processor
into a work, but let's put a duct-tape quickly at first.

Reported-and-tested-by: Dae R. Jeong <threeearcat@xxxxxxxxx>
Reported-by: syzbot+619d9f40141d826b097e@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 sound/core/seq/seq_virmidi.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

--- a/sound/core/seq/seq_virmidi.c
+++ b/sound/core/seq/seq_virmidi.c
@@ -163,6 +163,7 @@ static void snd_virmidi_output_trigger(s
 	int count, res;
 	unsigned char buf[32], *pbuf;
 	unsigned long flags;
+	bool check_resched = !in_atomic();
 
 	if (up) {
 		vmidi->trigger = 1;
@@ -200,6 +201,15 @@ static void snd_virmidi_output_trigger(s
 					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);