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

Takashi Iwai
2015-09-24T09:50:01Z

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@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@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@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
---
sound/pci/hda/hda_tegra.c | 30 +++++++++++++++++++++++++-----
1 file changed, 25 insertions(+), 5 deletions(-)

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