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

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Message-ID: <7ha8sddngh.fsf@deeprootsystems.com>
Date:	Wed, 23 Sep 2015 11:01:34 -0700
From:	Kevin Hilman <khilman@...nel.org>
To:	Thierry Reding <thierry.reding@...il.com>
Cc:	Takashi Iwai <tiwai@...e.com>, alsa-devel@...a-project.org,
	Tyler Baker <tyler.baker@...aro.org>,
	Jon Hunter <jonathanh@...dia.com>, linux-tegra@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] ALSA: hda - Avoid potential deadlock

Thierry Reding <thierry.reding@...il.com> writes:

> From: Thierry Reding <treding@...dia.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.

Tested-by: Kevin Hilman <khilman@...aro.org>

FWIW, I tested this on top of next-20150923, and it solves the boot
problem I'm seeing on tegra124-jetson-tk1 with module loading.

Kevin

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