Re: Sound/seq: hung task in odev_open - unuse callback sleeps under list_mutex

Re: Sound/seq: hung task in odev_open - unuse callback sleeps under list_mutex

[Takashi Iwai]

On Thu, 17 Sep 2026 04:41:31 +0200,
A Akhil wrote:
> 
> 
> To: tiwai@suse.com, perex@perex.cz
> Cc: linux-sound@vger.kernel.org, linux-kernel@vger.kernel.org,
>     syzkaller-bugs@googlegroups.com
> Subject: sound/seq: hung task in odev_open - unuse callback sleeps under
> list_mutex
> 
> Hi,
> 
> I've been looking at this syzbot report:
> 
>   INFO: task hung in odev_open (5)
>   https://syzkaller.appspot.com/bug?extid=825b7e3a03dd072c187f
> 
> The hang isn't really in the OSS code where it gets reported. syzbot only
> shows the task waiting on register_mutex in odev_open(); the one actually
> holding things up is this:
> 
>   odev_release                            takes register_mutex
>    snd_seq_oss_release / snd_seq_oss_reset
>     snd_seq_oss_synth_reset
>      snd_seq_oss_midi_close               takes mdev->open_mutex
>       snd_seq_ioctl_unsubscribe_port
>        snd_seq_port_disconnect
>         __delete_and_unsubscribe_port     called with grp->list_mutex held
>          midisynth_unuse                  seq_midi.c:298
>           snd_rawmidi_drain_output        sleeps 10*HZ per substream
>            schedule_timeout
> 
> delete_and_unsubscribe_port() holds grp->list_mutex for write while the
> unuse callback runs, and midisynth_unuse() ends up in
> snd_rawmidi_drain_output(), which waits 10*HZ per substream. With ~8
> substreams I measured close() taking 82 seconds. Anything calling
> snd_seq_port_connect() blocks on the same rwsem, and in the OSS path that
> connect runs under register_mutex, so every other odev_open() piles up
> behind it and the hung task detector fires.
> 
> I tried narrowing register_mutex in odev_release, and then open_mutex in
> snd_seq_oss_midi_close. Both build, both still hang - the wait just moves
> down a level each time, ending up in snd_seq_port_connect().
> 
> Moving unsubscribe_port() out of the rwsem looked wrong to me: grp->count
> is protected by it, and the comment above subscribe_port() says open and
> close are only invoked on the 0->1 and 1->0 transitions. If close ran
> unlocked, a concurrent subscribe could call open first.
> 
> That pairing also means an opener of the same port has to wait for a close
> in progress anyway, so I don't think lock narrowing can fix this - the
> teardown just has to stop taking tens of seconds.
> 
> So the drain has to stop blocking the teardown. I can see three ways to do
> that, but all of them change when or whether pending MIDI output gets
> flushed, and that's not something I want to decide on my own:
> 
>   - drop the output instead of draining it in midisynth_unuse()
>   - move the drain to a workqueue so the unuse callback doesn't sleep
>   - keep draining but cap the total wait
> 
> Which of those would you take? I can write it and test it.
> 
> (v7.3-rc3, 9b87fdc9af2f, qemu with dummy_hcd + raw-gadget. The syz repro
> alone didn't trigger it for me - I needed a second thread opening
> /dev/sequencer2 while another one closes it. That hits the hang in under a
> minute.)

IIUC, this is no real "hang" that locks up forever but just went over
threshold in mutex?  If so, we can avoid taking too long mutex like
the (totally untested) patch below?


thanks,

Takashi

-- 8< --
diff --git a/sound/core/seq/oss/seq_oss.c b/sound/core/seq/oss/seq_oss.c
index 2835576040ed..c790b1cd451a 100644
--- a/sound/core/seq/oss/seq_oss.c
+++ b/sound/core/seq/oss/seq_oss.c
@@ -132,13 +132,19 @@ static int
 odev_release(struct inode *inode, struct file *file)
 {
 	struct seq_oss_devinfo *dp;
+	int index;
 
 	dp = file->private_data;
 	if (!dp)
 		return 0;
 
-	guard(mutex)(&register_mutex);
+	scoped_guard(mutex, &register_mutex)
+		snd_seq_oss_detach(dp);
+	index = dp->index;
 	snd_seq_oss_release(dp);
+	scoped_guard(mutex, &register_mutex)
+		snd_seq_oss_detach_done(index);
+
 	return 0;
 }
 
@@ -149,6 +155,8 @@ odev_read(struct file *file, char __user *buf, size_t count, loff_t *offset)
 	dp = file->private_data;
 	if (snd_BUG_ON(!dp))
 		return -ENXIO;
+	if (dp->closing)
+		return -EBADFD;
 	return snd_seq_oss_read(dp, buf, count);
 }
 
@@ -160,6 +168,8 @@ odev_write(struct file *file, const char __user *buf, size_t count, loff_t *offs
 	dp = file->private_data;
 	if (snd_BUG_ON(!dp))
 		return -ENXIO;
+	if (dp->closing)
+		return -EBADFD;
 	return snd_seq_oss_write(dp, buf, count, file);
 }
 
diff --git a/sound/core/seq/oss/seq_oss_device.h b/sound/core/seq/oss/seq_oss_device.h
index 935cf3df0b30..ee318ea1a096 100644
--- a/sound/core/seq/oss/seq_oss_device.h
+++ b/sound/core/seq/oss/seq_oss_device.h
@@ -72,6 +72,7 @@ struct seq_oss_devinfo {
 	int cseq;	/* sequencer client number */
 	int port;	/* sequencer port number */
 	int queue;	/* sequencer queue number */
+	bool closing;
 
 	struct snd_seq_addr addr;	/* address of this device */
 
@@ -107,7 +108,9 @@ int snd_seq_oss_delete_client(void);
 
 /* device file interface */
 int snd_seq_oss_open(struct file *file, int level);
+void snd_seq_oss_detach(struct seq_oss_devinfo *dp);
 void snd_seq_oss_release(struct seq_oss_devinfo *dp);
+void snd_seq_oss_detach_done(int index);
 int snd_seq_oss_ioctl(struct seq_oss_devinfo *dp, unsigned int cmd, unsigned long arg);
 int snd_seq_oss_read(struct seq_oss_devinfo *dev, char __user *buf, int count);
 int snd_seq_oss_write(struct seq_oss_devinfo *dp, const char __user *buf, int count, struct file *opt);
diff --git a/sound/core/seq/oss/seq_oss_init.c b/sound/core/seq/oss/seq_oss_init.c
index 6586e07431c3..3aece0d2981a 100644
--- a/sound/core/seq/oss/seq_oss_init.c
+++ b/sound/core/seq/oss/seq_oss_init.c
@@ -33,6 +33,7 @@ static int system_port __ro_after_init = -1;
 static int num_clients;
 static struct seq_oss_devinfo *client_table[SNDRV_SEQ_OSS_MAX_CLIENTS];
 
+#define SEQ_OSS_DETACHED	((struct seq_oss_devinfo *)-1)
 
 /*
  * prototypes
@@ -396,14 +397,23 @@ free_devinfo(void *private)
 /*
  * close sequencer device
  */
+void snd_seq_oss_detach(struct seq_oss_devinfo *dp)
+{
+	dp->closing = true;
+	client_table[dp->index] = SEQ_OSS_DETACHED;
+	num_clients--;
+}
+
+void snd_seq_oss_detach_done(int index)
+{
+	client_table[index] = NULL;
+}
+
 void
 snd_seq_oss_release(struct seq_oss_devinfo *dp)
 {
 	int queue;
 
-	client_table[dp->index] = NULL;
-	num_clients--;
-
 	snd_seq_oss_reset(dp);
 
 	snd_seq_oss_synth_cleanup(dp);
@@ -475,7 +485,7 @@ snd_seq_oss_system_info_read(struct snd_info_buffer *buf)
 	for (i = 0; i < num_clients; i++) {
 		snd_iprintf(buf, "\nApplication %d: ", i);
 		dp = client_table[i];
-		if (!dp) {
+		if (!dp || dp == SEQ_OSS_DETACHED) {
 			snd_iprintf(buf, "*empty*\n");
 			continue;
 		}
diff --git a/sound/core/seq/oss/seq_oss_ioctl.c b/sound/core/seq/oss/seq_oss_ioctl.c
index f1a79776773f..2b4930a56a22 100644
--- a/sound/core/seq/oss/seq_oss_ioctl.c
+++ b/sound/core/seq/oss/seq_oss_ioctl.c
@@ -66,6 +66,9 @@ snd_seq_oss_ioctl(struct seq_oss_devinfo *dp, unsigned int cmd, unsigned long ca
 	void __user *arg = (void __user *)carg;
 	int __user *p = arg;
 
+	if (dp->closing)
+		return -EBADFD;
+
 	switch (cmd) {
 	case SNDCTL_TMR_TIMEBASE:
 	case SNDCTL_TMR_TEMPO:

Re: Sound/seq: hung task in odev_open - unuse callback sleeps under list_mutex

[A Akhil]

Hi Takashi,

Thanks for the quick patch. Right, it's not a deadlock, that was my
reading of it too. Nothing locks up permanently, it's just a wait long
enough to trip the detector. That's the reason I bothered with it though.
On the syzbot run the wait was over 140 s, and for the whole of that
anything else touching the sequencer is stuck behind it.

I ran it on the setup I used to find this in the first place. One thread
opening and closing /dev/sequencer2 while two others try to open it,
hung_task_timeout_secs=30, three runs each.

  variant                       hung tasks    worst opener block
  ---------------------------------------------------------------
  unpatched                     10, 10, 10    ~130 s
  your patch                    0, 0, 7       30.7 / 30.6 / 129.9 s
  yours + open_mutex narrowed   7, 0, 0       129.9 / 41.3 / 40.9 s
  drain bounded to 1s           0, 0, 0       20.6 / 18.4 / 21.8 s
  drop instead of drain         0, 0, 0       4.0 / 2.4 / 2.4 s

Your patch builds clean and clearly helps, two of the three runs were
completely quiet. The third still hung though, and when it does it's
coming from the opening side rather than the closing side.

  task 98:  odev_open                      holds register_mutex
             snd_seq_oss_open
              snd_seq_oss_synth_setup_midi
               snd_seq_oss_midi_open+0x8e  blocked on mdev->open_mutex
  tasks 97, 99: blocked on register_mutex owned by 98 -> hung task

An opener grabs register_mutex, blocks deeper down, and sits on it while
it waits.

I tried stacking my earlier open_mutex narrowing in snd_seq_oss_midi_close
on top of yours, so the closer would be holding neither lock over the
drain. Still hung 1 in 3, and the wait had just moved further down again,
this time into snd_seq_port_connect() on grp->list_mutex.

Three attempts at moving locks around now (mine, yours, both together) and
every one of them just shifts where the wait happens. I don't think the
locking is really the problem here. It's the 10*HZ per substream.

Bounding the drain to 1*HZ in midisynth_unuse() does stop the hangs, but
the cap is per substream and there are about twenty of them, so teardown
still runs ~20 s. Capping the total across the teardown would do better,
though that's more surgery.

The one that actually worked was swapping snd_rawmidi_drain_output() for
snd_rawmidi_drop_output() in midisynth_unuse(). ~2.4 s, nothing hung in
any run. Most of what's left there isn't even the data wait, it's the
msleep(50) per substream for the Tx FIFOs.

I did wonder about throwing the data away, so I went and looked. As far as
I can tell snd_rawmidi_drain_output() already drops the buffer once the
10*HZ expires, so the bytes are gone either way and dropping early only
costs whatever the device would have taken during the timeout. For a
device that isn't draining, which is the case that gets us here, that's
nothing. Tell me if I've misread that.

So, do you want me to send the drop version as a proper patch? Or would
you rather bound the drain, in which case I can try a total cap instead of
a per substream one. I can also test a revised version of yours if you'd
prefer to keep this inside seq_oss.

Thanks,
Akhil Arul




On Thu, 17 Sept 2026 at 16:37, Takashi Iwai <tiwai@suse.de> wrote:
>
> On Thu, 17 Sep 2026 04:41:31 +0200,
> A Akhil wrote:
> >
> >
> > To: tiwai@suse.com, perex@perex.cz
> > Cc: linux-sound@vger.kernel.org, linux-kernel@vger.kernel.org,
> >     syzkaller-bugs@googlegroups.com
> > Subject: sound/seq: hung task in odev_open - unuse callback sleeps under
> > list_mutex
> >
> > Hi,
> >
> > I've been looking at this syzbot report:
> >
> >   INFO: task hung in odev_open (5)
> >   https://syzkaller.appspot.com/bug?extid=825b7e3a03dd072c187f
> >
> > The hang isn't really in the OSS code where it gets reported. syzbot only
> > shows the task waiting on register_mutex in odev_open(); the one actually
> > holding things up is this:
> >
> >   odev_release                            takes register_mutex
> >    snd_seq_oss_release / snd_seq_oss_reset
> >     snd_seq_oss_synth_reset
> >      snd_seq_oss_midi_close               takes mdev->open_mutex
> >       snd_seq_ioctl_unsubscribe_port
> >        snd_seq_port_disconnect
> >         __delete_and_unsubscribe_port     called with grp->list_mutex held
> >          midisynth_unuse                  seq_midi.c:298
> >           snd_rawmidi_drain_output        sleeps 10*HZ per substream
> >            schedule_timeout
> >
> > delete_and_unsubscribe_port() holds grp->list_mutex for write while the
> > unuse callback runs, and midisynth_unuse() ends up in
> > snd_rawmidi_drain_output(), which waits 10*HZ per substream. With ~8
> > substreams I measured close() taking 82 seconds. Anything calling
> > snd_seq_port_connect() blocks on the same rwsem, and in the OSS path that
> > connect runs under register_mutex, so every other odev_open() piles up
> > behind it and the hung task detector fires.
> >
> > I tried narrowing register_mutex in odev_release, and then open_mutex in
> > snd_seq_oss_midi_close. Both build, both still hang - the wait just moves
> > down a level each time, ending up in snd_seq_port_connect().
> >
> > Moving unsubscribe_port() out of the rwsem looked wrong to me: grp->count
> > is protected by it, and the comment above subscribe_port() says open and
> > close are only invoked on the 0->1 and 1->0 transitions. If close ran
> > unlocked, a concurrent subscribe could call open first.
> >
> > That pairing also means an opener of the same port has to wait for a close
> > in progress anyway, so I don't think lock narrowing can fix this - the
> > teardown just has to stop taking tens of seconds.
> >
> > So the drain has to stop blocking the teardown. I can see three ways to do
> > that, but all of them change when or whether pending MIDI output gets
> > flushed, and that's not something I want to decide on my own:
> >
> >   - drop the output instead of draining it in midisynth_unuse()
> >   - move the drain to a workqueue so the unuse callback doesn't sleep
> >   - keep draining but cap the total wait
> >
> > Which of those would you take? I can write it and test it.
> >
> > (v7.3-rc3, 9b87fdc9af2f, qemu with dummy_hcd + raw-gadget. The syz repro
> > alone didn't trigger it for me - I needed a second thread opening
> > /dev/sequencer2 while another one closes it. That hits the hang in under a
> > minute.)
>
> IIUC, this is no real "hang" that locks up forever but just went over
> threshold in mutex?  If so, we can avoid taking too long mutex like
> the (totally untested) patch below?
>
>
> thanks,
>
> Takashi
>
> -- 8< --
> diff --git a/sound/core/seq/oss/seq_oss.c b/sound/core/seq/oss/seq_oss.c
> index 2835576040ed..c790b1cd451a 100644
> --- a/sound/core/seq/oss/seq_oss.c
> +++ b/sound/core/seq/oss/seq_oss.c
> @@ -132,13 +132,19 @@ static int
>  odev_release(struct inode *inode, struct file *file)
>  {
>         struct seq_oss_devinfo *dp;
> +       int index;
>
>         dp = file->private_data;
>         if (!dp)
>                 return 0;
>
> -       guard(mutex)(&register_mutex);
> +       scoped_guard(mutex, &register_mutex)
> +               snd_seq_oss_detach(dp);
> +       index = dp->index;
>         snd_seq_oss_release(dp);
> +       scoped_guard(mutex, &register_mutex)
> +               snd_seq_oss_detach_done(index);
> +
>         return 0;
>  }
>
> @@ -149,6 +155,8 @@ odev_read(struct file *file, char __user *buf, size_t count, loff_t *offset)
>         dp = file->private_data;
>         if (snd_BUG_ON(!dp))
>                 return -ENXIO;
> +       if (dp->closing)
> +               return -EBADFD;
>         return snd_seq_oss_read(dp, buf, count);
>  }
>
> @@ -160,6 +168,8 @@ odev_write(struct file *file, const char __user *buf, size_t count, loff_t *offs
>         dp = file->private_data;
>         if (snd_BUG_ON(!dp))
>                 return -ENXIO;
> +       if (dp->closing)
> +               return -EBADFD;
>         return snd_seq_oss_write(dp, buf, count, file);
>  }
>
> diff --git a/sound/core/seq/oss/seq_oss_device.h b/sound/core/seq/oss/seq_oss_device.h
> index 935cf3df0b30..ee318ea1a096 100644
> --- a/sound/core/seq/oss/seq_oss_device.h
> +++ b/sound/core/seq/oss/seq_oss_device.h
> @@ -72,6 +72,7 @@ struct seq_oss_devinfo {
>         int cseq;       /* sequencer client number */
>         int port;       /* sequencer port number */
>         int queue;      /* sequencer queue number */
> +       bool closing;
>
>         struct snd_seq_addr addr;       /* address of this device */
>
> @@ -107,7 +108,9 @@ int snd_seq_oss_delete_client(void);
>
>  /* device file interface */
>  int snd_seq_oss_open(struct file *file, int level);
> +void snd_seq_oss_detach(struct seq_oss_devinfo *dp);
>  void snd_seq_oss_release(struct seq_oss_devinfo *dp);
> +void snd_seq_oss_detach_done(int index);
>  int snd_seq_oss_ioctl(struct seq_oss_devinfo *dp, unsigned int cmd, unsigned long arg);
>  int snd_seq_oss_read(struct seq_oss_devinfo *dev, char __user *buf, int count);
>  int snd_seq_oss_write(struct seq_oss_devinfo *dp, const char __user *buf, int count, struct file *opt);
> diff --git a/sound/core/seq/oss/seq_oss_init.c b/sound/core/seq/oss/seq_oss_init.c
> index 6586e07431c3..3aece0d2981a 100644
> --- a/sound/core/seq/oss/seq_oss_init.c
> +++ b/sound/core/seq/oss/seq_oss_init.c
> @@ -33,6 +33,7 @@ static int system_port __ro_after_init = -1;
>  static int num_clients;
>  static struct seq_oss_devinfo *client_table[SNDRV_SEQ_OSS_MAX_CLIENTS];
>
> +#define SEQ_OSS_DETACHED       ((struct seq_oss_devinfo *)-1)
>
>  /*
>   * prototypes
> @@ -396,14 +397,23 @@ free_devinfo(void *private)
>  /*
>   * close sequencer device
>   */
> +void snd_seq_oss_detach(struct seq_oss_devinfo *dp)
> +{
> +       dp->closing = true;
> +       client_table[dp->index] = SEQ_OSS_DETACHED;
> +       num_clients--;
> +}
> +
> +void snd_seq_oss_detach_done(int index)
> +{
> +       client_table[index] = NULL;
> +}
> +
>  void
>  snd_seq_oss_release(struct seq_oss_devinfo *dp)
>  {
>         int queue;
>
> -       client_table[dp->index] = NULL;
> -       num_clients--;
> -
>         snd_seq_oss_reset(dp);
>
>         snd_seq_oss_synth_cleanup(dp);
> @@ -475,7 +485,7 @@ snd_seq_oss_system_info_read(struct snd_info_buffer *buf)
>         for (i = 0; i < num_clients; i++) {
>                 snd_iprintf(buf, "\nApplication %d: ", i);
>                 dp = client_table[i];
> -               if (!dp) {
> +               if (!dp || dp == SEQ_OSS_DETACHED) {
>                         snd_iprintf(buf, "*empty*\n");
>                         continue;
>                 }
> diff --git a/sound/core/seq/oss/seq_oss_ioctl.c b/sound/core/seq/oss/seq_oss_ioctl.c
> index f1a79776773f..2b4930a56a22 100644
> --- a/sound/core/seq/oss/seq_oss_ioctl.c
> +++ b/sound/core/seq/oss/seq_oss_ioctl.c
> @@ -66,6 +66,9 @@ snd_seq_oss_ioctl(struct seq_oss_devinfo *dp, unsigned int cmd, unsigned long ca
>         void __user *arg = (void __user *)carg;
>         int __user *p = arg;
>
> +       if (dp->closing)
> +               return -EBADFD;
> +
>         switch (cmd) {
>         case SNDCTL_TMR_TIMEBASE:
>         case SNDCTL_TMR_TEMPO:

Re: Sound/seq: hung task in odev_open - unuse callback sleeps under list_mutex

[Takashi Iwai]

On Thu, 17 Sep 2026 16:11:27 +0200,
A Akhil wrote:
> 
> Hi Takashi,
> 
> Thanks for the quick patch. Right, it's not a deadlock, that was my
> reading of it too. Nothing locks up permanently, it's just a wait long
> enough to trip the detector. That's the reason I bothered with it though.
> On the syzbot run the wait was over 140 s, and for the whole of that
> anything else touching the sequencer is stuck behind it.
> 
> I ran it on the setup I used to find this in the first place. One thread
> opening and closing /dev/sequencer2 while two others try to open it,
> hung_task_timeout_secs=30, three runs each.
> 
>   variant                       hung tasks    worst opener block
>   ---------------------------------------------------------------
>   unpatched                     10, 10, 10    ~130 s
>   your patch                    0, 0, 7       30.7 / 30.6 / 129.9 s
>   yours + open_mutex narrowed   7, 0, 0       129.9 / 41.3 / 40.9 s
>   drain bounded to 1s           0, 0, 0       20.6 / 18.4 / 21.8 s
>   drop instead of drain         0, 0, 0       4.0 / 2.4 / 2.4 s
> 
> Your patch builds clean and clearly helps, two of the three runs were
> completely quiet. The third still hung though, and when it does it's
> coming from the opening side rather than the closing side.
> 
>   task 98:  odev_open                      holds register_mutex
>              snd_seq_oss_open
>               snd_seq_oss_synth_setup_midi
>                snd_seq_oss_midi_open+0x8e  blocked on mdev->open_mutex
>   tasks 97, 99: blocked on register_mutex owned by 98 -> hung task
> 
> An opener grabs register_mutex, blocks deeper down, and sits on it while
> it waits.
> 
> I tried stacking my earlier open_mutex narrowing in snd_seq_oss_midi_close
> on top of yours, so the closer would be holding neither lock over the
> drain. Still hung 1 in 3, and the wait had just moved further down again,
> this time into snd_seq_port_connect() on grp->list_mutex.
> 
> Three attempts at moving locks around now (mine, yours, both together) and
> every one of them just shifts where the wait happens. I don't think the
> locking is really the problem here. It's the 10*HZ per substream.
> 
> Bounding the drain to 1*HZ in midisynth_unuse() does stop the hangs, but
> the cap is per substream and there are about twenty of them, so teardown
> still runs ~20 s. Capping the total across the teardown would do better,
> though that's more surgery.
> 
> The one that actually worked was swapping snd_rawmidi_drain_output() for
> snd_rawmidi_drop_output() in midisynth_unuse(). ~2.4 s, nothing hung in
> any run. Most of what's left there isn't even the data wait, it's the
> msleep(50) per substream for the Tx FIFOs.
> 
> I did wonder about throwing the data away, so I went and looked. As far as
> I can tell snd_rawmidi_drain_output() already drops the buffer once the
> 10*HZ expires, so the bytes are gone either way and dropping early only
> costs whatever the device would have taken during the timeout. For a
> device that isn't draining, which is the case that gets us here, that's
> nothing. Tell me if I've misread that.
> 
> So, do you want me to send the drop version as a proper patch? Or would
> you rather bound the drain, in which case I can try a total cap instead of
> a per substream one. I can also test a revised version of yours if you'd
> prefer to keep this inside seq_oss.

Hm, dropping isn't optimal, as that's a clear behavior change; there
can be pending bytes even in the real use case.

Actually, shortening the drain limit would be an easier way.
Practically seen, 1 second should be enough for the real hardware.
If this is enough for the syzkaller report, we can take it quickly.

Another option would be to offload the snd_seq_kernel_client_ctl()
calls in seq_oss_midi.c snd_seq_oss_midi_open() and close() to a work,
as you pointed out previously.  This would be relatively safe, I
suppose.

In general, I'd like to avoid touching too much outside the OSS
emulation layer.  The shortening of the drain limit would be OK, but
restructuring else isn't preferred.

In anyway, if you can pitch some good workaround, I'll happily take.

(But I'm going to be off from tomorrow, so it'll be continued after my
vacation.)


thanks,

Takashi

Re: Sound/seq: hung task in odev_open - unuse callback sleeps under list_mutex

[A Akhil]

Hi Takashi,

Fair enough on dropping, I'll leave that alone.

I'd actually already tried the 1 second limit before your mail came in, so
the short answer is yes, it holds up. Same rig as before, one thread
opening and closing /dev/sequencer2 while two others open it, three runs
each:

  unpatched              hung 10, 10, 10    worst block ~130 s
  your patch             hung  0,  0,  7    30.7 / 30.6 / 129.9 s
  drain capped at 1*HZ   hung  0,  0,  0    20.6 / 18.4 / 21.8 s
  progress detection     hung  0,  0,  0    11.0 / 11.1 / 11.6 s  <- new

I was running with hung_task_timeout_secs=30 to make the failures turn up
faster, so at the 120 s default the 1*HZ cap has plenty of room.

The last row is new, and it's the one I'd rather pitch. Rather than
picking a constant, it just checks whether the device is draining at all:
wait in HZ/5 slices and give up once avail stops changing, with 10*HZ
still there as the outer bound. A device that works drains completely as
before, nothing gets truncated, and a stalled one gets spotted in about
400 ms. Comes out at roughly half the teardown time of the 1 s cap here,
and it's about 20 lines inside snd_rawmidi_drain_output(), nothing
outside it.

One note in case you do go with the cap. MIDI is 3125 bytes/s and the
buffer is PAGE_SIZE, so a completely full 4K buffer needs 1.31 s to get
out. 1*HZ would clip that, and 10*HZ already clips a full buffer on 64K
page kernels. Unlikely to bite in practice since the buffer is rarely
full at close, but it's the reason I went looking for something that
doesn't depend on a fixed number.

Also a correction to my last mail: I said about twenty substreams, which
was wrong. It's one rawmidi device, midiC0D0, and the teardown just calls
the drain repeatedly, around 31 times per cycle.

If either of these looks fine to you I'll send it as a proper patch. And
if I find something better in the meantime I'll pitch that instead.

Enjoy your vacation.

Thanks,
Akhil Arul


On Thu, 17 Sept 2026 at 20:25, Takashi Iwai <tiwai@suse.de> wrote:
>
> On Thu, 17 Sep 2026 16:11:27 +0200,
> A Akhil wrote:
> >
> > Hi Takashi,
> >
> > Thanks for the quick patch. Right, it's not a deadlock, that was my
> > reading of it too. Nothing locks up permanently, it's just a wait long
> > enough to trip the detector. That's the reason I bothered with it though.
> > On the syzbot run the wait was over 140 s, and for the whole of that
> > anything else touching the sequencer is stuck behind it.
> >
> > I ran it on the setup I used to find this in the first place. One thread
> > opening and closing /dev/sequencer2 while two others try to open it,
> > hung_task_timeout_secs=30, three runs each.
> >
> >   variant                       hung tasks    worst opener block
> >   ---------------------------------------------------------------
> >   unpatched                     10, 10, 10    ~130 s
> >   your patch                    0, 0, 7       30.7 / 30.6 / 129.9 s
> >   yours + open_mutex narrowed   7, 0, 0       129.9 / 41.3 / 40.9 s
> >   drain bounded to 1s           0, 0, 0       20.6 / 18.4 / 21.8 s
> >   drop instead of drain         0, 0, 0       4.0 / 2.4 / 2.4 s
> >
> > Your patch builds clean and clearly helps, two of the three runs were
> > completely quiet. The third still hung though, and when it does it's
> > coming from the opening side rather than the closing side.
> >
> >   task 98:  odev_open                      holds register_mutex
> >              snd_seq_oss_open
> >               snd_seq_oss_synth_setup_midi
> >                snd_seq_oss_midi_open+0x8e  blocked on mdev->open_mutex
> >   tasks 97, 99: blocked on register_mutex owned by 98 -> hung task
> >
> > An opener grabs register_mutex, blocks deeper down, and sits on it while
> > it waits.
> >
> > I tried stacking my earlier open_mutex narrowing in snd_seq_oss_midi_close
> > on top of yours, so the closer would be holding neither lock over the
> > drain. Still hung 1 in 3, and the wait had just moved further down again,
> > this time into snd_seq_port_connect() on grp->list_mutex.
> >
> > Three attempts at moving locks around now (mine, yours, both together) and
> > every one of them just shifts where the wait happens. I don't think the
> > locking is really the problem here. It's the 10*HZ per substream.
> >
> > Bounding the drain to 1*HZ in midisynth_unuse() does stop the hangs, but
> > the cap is per substream and there are about twenty of them, so teardown
> > still runs ~20 s. Capping the total across the teardown would do better,
> > though that's more surgery.
> >
> > The one that actually worked was swapping snd_rawmidi_drain_output() for
> > snd_rawmidi_drop_output() in midisynth_unuse(). ~2.4 s, nothing hung in
> > any run. Most of what's left there isn't even the data wait, it's the
> > msleep(50) per substream for the Tx FIFOs.
> >
> > I did wonder about throwing the data away, so I went and looked. As far as
> > I can tell snd_rawmidi_drain_output() already drops the buffer once the
> > 10*HZ expires, so the bytes are gone either way and dropping early only
> > costs whatever the device would have taken during the timeout. For a
> > device that isn't draining, which is the case that gets us here, that's
> > nothing. Tell me if I've misread that.
> >
> > So, do you want me to send the drop version as a proper patch? Or would
> > you rather bound the drain, in which case I can try a total cap instead of
> > a per substream one. I can also test a revised version of yours if you'd
> > prefer to keep this inside seq_oss.
>
> Hm, dropping isn't optimal, as that's a clear behavior change; there
> can be pending bytes even in the real use case.
>
> Actually, shortening the drain limit would be an easier way.
> Practically seen, 1 second should be enough for the real hardware.
> If this is enough for the syzkaller report, we can take it quickly.
>
> Another option would be to offload the snd_seq_kernel_client_ctl()
> calls in seq_oss_midi.c snd_seq_oss_midi_open() and close() to a work,
> as you pointed out previously.  This would be relatively safe, I
> suppose.
>
> In general, I'd like to avoid touching too much outside the OSS
> emulation layer.  The shortening of the drain limit would be OK, but
> restructuring else isn't preferred.
>
> In anyway, if you can pitch some good workaround, I'll happily take.
>
> (But I'm going to be off from tomorrow, so it'll be continued after my
> vacation.)
>
>
> thanks,
>
> Takashi

Re: Sound/seq: hung task in odev_open - unuse callback sleeps under list_mutex

[Takashi Iwai]

On Thu, 17 Sep 2026 17:42:36 +0200,
A Akhil wrote:
> 
> Hi Takashi,
> 
> Fair enough on dropping, I'll leave that alone.
> 
> I'd actually already tried the 1 second limit before your mail came in, so
> the short answer is yes, it holds up. Same rig as before, one thread
> opening and closing /dev/sequencer2 while two others open it, three runs
> each:
> 
>   unpatched              hung 10, 10, 10    worst block ~130 s
>   your patch             hung  0,  0,  7    30.7 / 30.6 / 129.9 s
>   drain capped at 1*HZ   hung  0,  0,  0    20.6 / 18.4 / 21.8 s
>   progress detection     hung  0,  0,  0    11.0 / 11.1 / 11.6 s  <- new
> 
> I was running with hung_task_timeout_secs=30 to make the failures turn up
> faster, so at the 120 s default the 1*HZ cap has plenty of room.
> 
> The last row is new, and it's the one I'd rather pitch. Rather than
> picking a constant, it just checks whether the device is draining at all:
> wait in HZ/5 slices and give up once avail stops changing, with 10*HZ
> still there as the outer bound. A device that works drains completely as
> before, nothing gets truncated, and a stalled one gets spotted in about
> 400 ms. Comes out at roughly half the teardown time of the 1 s cap here,
> and it's about 20 lines inside snd_rawmidi_drain_output(), nothing
> outside it.

Sounds like a good idea.

> One note in case you do go with the cap. MIDI is 3125 bytes/s and the
> buffer is PAGE_SIZE, so a completely full 4K buffer needs 1.31 s to get
> out. 1*HZ would clip that, and 10*HZ already clips a full buffer on 64K
> page kernels. Unlikely to bite in practice since the buffer is rarely
> full at close, but it's the reason I went looking for something that
> doesn't depend on a fixed number.

Fair enough.

> Also a correction to my last mail: I said about twenty substreams, which
> was wrong. It's one rawmidi device, midiC0D0, and the teardown just calls
> the drain repeatedly, around 31 times per cycle.
> 
> If either of these looks fine to you I'll send it as a proper patch. And
> if I find something better in the meantime I'll pitch that instead.

The idea with an adaptive timeout looks promising, so I'd like to
check this primarily, unless it becomes too complex.

> Enjoy your vacation.

Thanks!


Takashi

> 
> Thanks,
> Akhil Arul
> 
> 
> On Thu, 17 Sept 2026 at 20:25, Takashi Iwai <tiwai@suse.de> wrote:
> >
> > On Thu, 17 Sep 2026 16:11:27 +0200,
> > A Akhil wrote:
> > >
> > > Hi Takashi,
> > >
> > > Thanks for the quick patch. Right, it's not a deadlock, that was my
> > > reading of it too. Nothing locks up permanently, it's just a wait long
> > > enough to trip the detector. That's the reason I bothered with it though.
> > > On the syzbot run the wait was over 140 s, and for the whole of that
> > > anything else touching the sequencer is stuck behind it.
> > >
> > > I ran it on the setup I used to find this in the first place. One thread
> > > opening and closing /dev/sequencer2 while two others try to open it,
> > > hung_task_timeout_secs=30, three runs each.
> > >
> > >   variant                       hung tasks    worst opener block
> > >   ---------------------------------------------------------------
> > >   unpatched                     10, 10, 10    ~130 s
> > >   your patch                    0, 0, 7       30.7 / 30.6 / 129.9 s
> > >   yours + open_mutex narrowed   7, 0, 0       129.9 / 41.3 / 40.9 s
> > >   drain bounded to 1s           0, 0, 0       20.6 / 18.4 / 21.8 s
> > >   drop instead of drain         0, 0, 0       4.0 / 2.4 / 2.4 s
> > >
> > > Your patch builds clean and clearly helps, two of the three runs were
> > > completely quiet. The third still hung though, and when it does it's
> > > coming from the opening side rather than the closing side.
> > >
> > >   task 98:  odev_open                      holds register_mutex
> > >              snd_seq_oss_open
> > >               snd_seq_oss_synth_setup_midi
> > >                snd_seq_oss_midi_open+0x8e  blocked on mdev->open_mutex
> > >   tasks 97, 99: blocked on register_mutex owned by 98 -> hung task
> > >
> > > An opener grabs register_mutex, blocks deeper down, and sits on it while
> > > it waits.
> > >
> > > I tried stacking my earlier open_mutex narrowing in snd_seq_oss_midi_close
> > > on top of yours, so the closer would be holding neither lock over the
> > > drain. Still hung 1 in 3, and the wait had just moved further down again,
> > > this time into snd_seq_port_connect() on grp->list_mutex.
> > >
> > > Three attempts at moving locks around now (mine, yours, both together) and
> > > every one of them just shifts where the wait happens. I don't think the
> > > locking is really the problem here. It's the 10*HZ per substream.
> > >
> > > Bounding the drain to 1*HZ in midisynth_unuse() does stop the hangs, but
> > > the cap is per substream and there are about twenty of them, so teardown
> > > still runs ~20 s. Capping the total across the teardown would do better,
> > > though that's more surgery.
> > >
> > > The one that actually worked was swapping snd_rawmidi_drain_output() for
> > > snd_rawmidi_drop_output() in midisynth_unuse(). ~2.4 s, nothing hung in
> > > any run. Most of what's left there isn't even the data wait, it's the
> > > msleep(50) per substream for the Tx FIFOs.
> > >
> > > I did wonder about throwing the data away, so I went and looked. As far as
> > > I can tell snd_rawmidi_drain_output() already drops the buffer once the
> > > 10*HZ expires, so the bytes are gone either way and dropping early only
> > > costs whatever the device would have taken during the timeout. For a
> > > device that isn't draining, which is the case that gets us here, that's
> > > nothing. Tell me if I've misread that.
> > >
> > > So, do you want me to send the drop version as a proper patch? Or would
> > > you rather bound the drain, in which case I can try a total cap instead of
> > > a per substream one. I can also test a revised version of yours if you'd
> > > prefer to keep this inside seq_oss.
> >
> > Hm, dropping isn't optimal, as that's a clear behavior change; there
> > can be pending bytes even in the real use case.
> >
> > Actually, shortening the drain limit would be an easier way.
> > Practically seen, 1 second should be enough for the real hardware.
> > If this is enough for the syzkaller report, we can take it quickly.
> >
> > Another option would be to offload the snd_seq_kernel_client_ctl()
> > calls in seq_oss_midi.c snd_seq_oss_midi_open() and close() to a work,
> > as you pointed out previously.  This would be relatively safe, I
> > suppose.
> >
> > In general, I'd like to avoid touching too much outside the OSS
> > emulation layer.  The shortening of the drain limit would be OK, but
> > restructuring else isn't preferred.
> >
> > In anyway, if you can pitch some good workaround, I'll happily take.
> >
> > (But I'm going to be off from tomorrow, so it'll be continued after my
> > vacation.)
> >
> >
> > thanks,
> >
> > Takashi