On Thu, 30 Sep 2021 08:31:56 +0200,
Takashi Iwai wrote:
>
> On Wed, 29 Sep 2021 18:56:32 +0200,
> John Keeping wrote:
> >
> > On Wed, 29 Sep 2021 17:28:57 +0200
> > Takashi Iwai <tiwai@xxxxxxx> wrote:
> >
> > > On Wed, 29 Sep 2021 17:17:58 +0200,
> > > John Keeping wrote:
> > > >
> > > > On Wed, 29 Sep 2021 16:51:47 +0200
> > > > Takashi Iwai <tiwai@xxxxxxx> wrote:
> > > >
> > > > > On Wed, 29 Sep 2021 13:36:20 +0200,
> > > > > John Keeping wrote:
> > > > > >
> > > > > > If the sequencer device outlives the rawmidi device, then
> > > > > > snd_rawmidi_dev_seq_free() will run after release_rawmidi_device() has
> > > > > > freed the snd_rawmidi structure.
> > > > > >
> > > > > > This can easily be reproduced with CONFIG_DEBUG_KOBJECT_RELEASE.
> > > > > >
> > > > > > Keep a reference to the rawmidi device until the sequencer has been
> > > > > > destroyed in order to avoid this.
> > > > > >
> > > > > > Signed-off-by: John Keeping <john@xxxxxxxxxxxx>
> > > > >
> > > > > Thanks for the patch. I wonder, though, how this could be triggered.
> > > > > Is this the case where the connected sequencer device is being used
> > > > > while the sound card gets released? Or is it something else?
> > > >
> > > > I'm not sure if it's possible to trigger via the ALSA API; I haven't
> > > > found a route that can trigger it, but that doesn't mean there isn't
> > > > one :-)
> > > >
> > > > Mostly this is useful to make CONFIG_DEBUG_KOBJECT_RELEASE cleaner.
> > >
> > > Hm, then could you check whether the patch below papers over it
> > > instead?
> >
> > No, this patch doesn't solve it. The issue is that the effect of the
> > final device_put() is delayed from the time it is called and there is no
> > way to guarantee the ordering without ensuring the sequencer has been
> > destroyed before the final reference to the rawmidi device is put.
> >
> > Both of the functions involved are called from the core
> > device::release() hook.
> >
> > I'm using the patch below to easily check that the sequencer has been
> > freed before the rawmidi data. This can easily be triggered by
> > unplugging a USB MIDI device (it's not 100% since the kobject release
> > delays are random).
>
> Hm, it's strange. I suppose you're *not* using the MIDI device,
> right?
>
> The release path for the USB-audio driver is:
> usb_audio_disconnect() ->
> snd_card_free_when_closed() ->
> release_card_device() (via put_device(&card->card_dev)) ->
> snd_card_do_free()
>
> And here in snd_card_do_free(), the snd_device free-callback chains
> are called at the beginning (snd_device_free_all()).
> As it's executed in a reverse loop, snd_rawmidi_dev_seq_free() shall
> be called before snd_rawmidi_dev_free(). Since the final put_device()
> for the rawmidi device is called in the latter function, the device
> release must not happen before snd_rawmidi_dev_seq_free()...
Correction: now I finally understood what I misunderstood.
Although the snd_device call chain mentioned above itself is correct,
the snd_rawmidi_dev_seq_free() function isn't called directly from the
snd_device chain, but it's rater the own private_free of
snd_seq_device object. That is, the call of snd_seq_device
private_free is done in a wrong place; it should be called in the
snd_device call chain instead of the device release.
A fix patch is something like below. Could you check whether this
fixes the problem?
thanks,
Takashi
--- a/sound/core/seq_device.c
+++ b/sound/core/seq_device.c
@@ -156,6 +156,8 @@ static int snd_seq_device_dev_free(struct snd_device *device)
struct snd_seq_device *dev = device->device_data;
cancel_autoload_drivers();
+ if (dev->private_free)
+ dev->private_free(dev);
put_device(&dev->dev);
return 0;
}
@@ -183,11 +185,7 @@ static int snd_seq_device_dev_disconnect(struct snd_device *device)
static void snd_seq_dev_release(struct device *dev)
{
- struct snd_seq_device *sdev = to_seq_dev(dev);
-
- if (sdev->private_free)
- sdev->private_free(sdev);
- kfree(sdev);
+ kfree(to_seq_dev(dev));
}
/*
