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

Takashi Iwai
2015-09-19T16:42:30Z

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.

This looks like a sane cleanup at a glance, but I can't review in
details now because I'm traveling on vacation. I'll take a closer
look in the next week.

thanks,

Takashi

>
> Thierry
>
> Thierry Reding (3):
> ALSA: hda/hdmi - Add missing MODALIAS information
> ALSA: hda - Advertise MODALIAS in uevent
> ALSA: hda: Do not rely on explicit module loading
>
> sound/hda/hda_bus_type.c | 12 +++++++
> sound/pci/hda/hda_bind.c | 80 ----------------------------------------------
> sound/pci/hda/patch_hdmi.c | 3 ++
> 3 files changed, 15 insertions(+), 80 deletions(-)
>
> --
> 2.5.0
>
>