ALSA: seq: oss: Avoid mutex lock for a long-time ioctl

[ Upstream commit 2759caad2600d503c3b0ed800e7e03d2cd7a4c05 ]

Recently we applied a fix to cover the whole OSS sequencer ioctls with
the mutex for dealing with the possible races.  This works fine in
general, but in theory, this may lead to unexpectedly long stall if an
ioctl like SNDCTL_SEQ_SYNC is issued and an event with the far future
timestamp was queued.

For fixing such a potential stall, this patch changes the mutex lock
applied conditionally excluding such an ioctl command.  Also, change
the mutex_lock() with the interruptible version for user to allow
escaping from the big-hammer mutex.

Fixes: 80982c7e834e ("ALSA: seq: oss: Serialize ioctls")
Suggested-by: Pavel Machek <pavel@ucw.cz>
Link: https://lore.kernel.org/r/20200922083856.28572-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/sound/core/seq/oss/seq_oss.c b/sound/core/seq/oss/seq_oss.c
index 8044775999eda68f4b8942a02ed383c572116f05..4d1548b951c41bc42f84e6f663f514e8f3a92ca0 100644 (file)
--- a/sound/core/seq/oss/seq_oss.c
+++ b/sound/core/seq/oss/seq_oss.c
@@ -186,9 +186,12 @@ odev_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
        if (snd_BUG_ON(!dp))
                return -ENXIO;
 
-       mutex_lock(&register_mutex);
+       if (cmd != SNDCTL_SEQ_SYNC &&
+           mutex_lock_interruptible(&register_mutex))
+               return -ERESTARTSYS;
        rc = snd_seq_oss_ioctl(dp, cmd, arg);
-       mutex_unlock(&register_mutex);
+       if (cmd != SNDCTL_SEQ_SYNC)
+               mutex_unlock(&register_mutex);
        return rc;
 }