[alsa-devel] [PATCH 0/3] ALSA: hda - Avoid potential deadlock

[Takashi Iwai]

On Thu, 24 Sep 2015 12:50:10 +0200,
Thierry Reding wrote:
> 
> On Thu, Sep 24, 2015 at 11:49:57AM +0200, Takashi Iwai wrote:
> > On Wed, 23 Sep 2015 11:03:44 +0200,
> > Takashi Iwai wrote:
> > > 
> > > On Thu, 17 Sep 2015 12:00:03 +0200,
> > > Thierry Reding wrote:
> > > > 
> > > > From: Thierry Reding <treding at nvidia.com>
> > > > 
> > > > The Tegra HDA controller driver committed in v3.16 causes deadlocks when
> > > > loaded as a module. The reason is that the driver core will lock the HDA
> > > > controller device upon calling its probe callback and the probe callback
> > > > then goes on to create child devices for detected codecs and loads their
> > > > modules via a request_module() call. This is problematic because the new
> > > > driver will immediately be bound to the device, which will in turn cause
> > > > the parent of the codec device (the HDA controller device) to be locked
> > > > again, causing a deadlock.
> > > > 
> > > > This problem seems to have been present since the modularization of the
> > > > HD-audio driver in commit 1289e9e8b42f ("ALSA: hda - Modularize HD-audio
> > > > driver"). On Intel platforms this has been worked around by splitting up
> > > > the probe sequence into a synchronous and an asynchronous part where the
> > > > request_module() calls are asynchronous and hence avoid the deadlock.
> > > > 
> > > > An alternative proposal is provided in this series of patches. Rather
> > > > than relying on explicit request_module() calls to load kernel modules
> > > > for HDA codec drivers, this implements a uevent callback for the HDA bus
> > > > to advertises the MODALIAS information to the userspace helper.
> > > > 
> > > > Effectively this results in the same modules being loaded, but it uses
> > > > the more canonical infrastructure to perform this. Deferring the module
> > > > loading to userspace removes the need for the explicit request_module()
> > > > calls and works around the recursive locking issue because both drivers
> > > > will be bound from separate contexts.
> > > 
> > > While this looks definitely like the right direction to go, I'm afraid
> > > that this will give a few major regressions.  First off, there is no
> > > way to bind with the generic codec driver.  There are two generic
> > > drivers, one for HDMI/DP and one for normal audio.  Binding to them is
> > > judged by parsing the codec widgets whether they are digital-only.
> > > So, either user-space or kernel needs to parse the codec widgets
> > > beforehand.  If we rip off all binding magic as in your patch, this
> > > has to be done by udev.  With the sysfs stuff, now it should be
> > > possible, but this would break the existing system.
> > > 
> > > Another possible regression is the matching with the vendor-only
> > > alias.  Maybe the current wildcard works, but we need to double
> > > check.
> > > 
> > > So, unless these are addressed, I think we need another quick band-aid
> > > over snd-hda-tegra just doing the async probe like snd-hda-intel.
> > 
> > Does the patch below work?  I only did a quick compile test.
> > 
> > 
> > thanks,
> > 
> > Takashi
> > 
> > -- 8< --
> > From: Takashi Iwai <tiwai at suse.de>
> > Subject: [PATCH] ALSA: hda/tegra - async probe for avoiding module loading
> >  deadlock
> > 
> > The Tegra HD-audio controller driver causes deadlocks when loaded as a
> > module since the driver invokes request_module() at binding with the
> > codec driver.  This patch works around it by deferring the probe in a
> > work like Intel HD-audio controller driver does.  Although hovering
> > the codec probe stuff into udev would be a better solution, it may
> > cause other regressions, so let's try this band-aid fix until the more
> > proper solution gets landed.
> > 
> > Reported-by: Thierry Reding <treding at nvidia.com>
> > Cc: <stable at vger.kernel.org>
> > Signed-off-by: Takashi Iwai <tiwai at suse.de>
> > ---
> >  sound/pci/hda/hda_tegra.c | 30 +++++++++++++++++++++++++-----
> >  1 file changed, 25 insertions(+), 5 deletions(-)
> 
> Yes, that fixes the hang that I was seeing:
> 
> Tested-by: Thierry Reding <treding at nvidia.com>

Thanks!  I'll queue this for the next pull request.

> As a matter of fact this resembles a patch that Jon had worked on to
> solve this. I'm slightly concerned that merging a band-aid like this
> is going to remove any incentive to fix this properly, though.

Yeah, it's neither elegant nor cleaner solution but it's certainly
safer.


Takashi

> Thierry
> 
> > diff --git a/sound/pci/hda/hda_tegra.c b/sound/pci/hda/hda_tegra.c
> > index 477742cb70a2..58c0aad37284 100644
> > --- a/sound/pci/hda/hda_tegra.c
> > +++ b/sound/pci/hda/hda_tegra.c
> > @@ -73,6 +73,7 @@ struct hda_tegra {
> >  	struct clk *hda2codec_2x_clk;
> >  	struct clk *hda2hdmi_clk;
> >  	void __iomem *regs;
> > +	struct work_struct probe_work;
> >  };
> >  
> >  #ifdef CONFIG_PM
> > @@ -294,7 +295,9 @@ static int hda_tegra_dev_disconnect(struct snd_device *device)
> >  static int hda_tegra_dev_free(struct snd_device *device)
> >  {
> >  	struct azx *chip = device->device_data;
> > +	struct hda_tegra *hda = container_of(chip, struct hda_tegra, chip);
> >  
> > +	cancel_work_sync(&hda->probe_work);
> >  	if (azx_bus(chip)->chip_init) {
> >  		azx_stop_all_streams(chip);
> >  		azx_stop_chip(chip);
> > @@ -426,6 +429,9 @@ static int hda_tegra_first_init(struct azx *chip, struct platform_device *pdev)
> >  /*
> >   * constructor
> >   */
> > +
> > +static void hda_tegra_probe_work(struct work_struct *work);
> > +
> >  static int hda_tegra_create(struct snd_card *card,
> >  			    unsigned int driver_caps,
> >  			    struct hda_tegra *hda)
> > @@ -452,6 +458,8 @@ static int hda_tegra_create(struct snd_card *card,
> >  	chip->single_cmd = false;
> >  	chip->snoop = true;
> >  
> > +	INIT_WORK(&hda->probe_work, hda_tegra_probe_work);
> > +
> >  	err = azx_bus_init(chip, NULL, &hda_tegra_io_ops);
> >  	if (err < 0)
> >  		return err;
> > @@ -499,6 +507,21 @@ static int hda_tegra_probe(struct platform_device *pdev)
> >  	card->private_data = chip;
> >  
> >  	dev_set_drvdata(&pdev->dev, card);
> > +	schedule_work(&hda->probe_work);
> > +
> > +	return 0;
> > +
> > +out_free:
> > +	snd_card_free(card);
> > +	return err;
> > +}
> > +
> > +static void hda_tegra_probe_work(struct work_struct *work)
> > +{
> > +	struct hda_tegra *hda = container_of(work, struct hda_tegra, probe_work);
> > +	struct azx *chip = &hda->chip;
> > +	struct platform_device *pdev = to_platform_device(hda->dev);
> > +	int err;
> >  
> >  	err = hda_tegra_first_init(chip, pdev);
> >  	if (err < 0)
> > @@ -520,11 +543,8 @@ static int hda_tegra_probe(struct platform_device *pdev)
> >  	chip->running = 1;
> >  	snd_hda_set_power_save(&chip->bus, power_save * 1000);
> >  
> > -	return 0;
> > -
> > -out_free:
> > -	snd_card_free(card);
> > -	return err;
> > + out_free:
> > +	return; /* no error return from async probe */
> >  }
> >  
> >  static int hda_tegra_remove(struct platform_device *pdev)
> > -- 
> > 2.5.1
> >