[alsa-devel] [PATCH] ASoC: core: Fix race in dapm_power_widgets

[Peter Ujfalusi]

On Thursday 04 November 2010 20:08:58 ext Mark Brown wrote:
> Right, that's what I was thinking - I'm mostly concerned that we need to
> have the locking further up the stack rather than further down.  This
> feels like a band aid which fixes the symptom (the list pointers getting
> corrupted) but it smells like we may have something further up the stack
> which can still create issues.  For example, if both DAPM and something
> else manipulate the register map simultaneously we could get R/M/W
> issues.

Well, I think if we want to have one place to use a lock, this is the highest we 
can go.
But I did figured out the reason for the crash.
In my machine driver I had a kcontrol (ENUM_EXT) for muting the speaker.
In the set callback I used snd_soc_dapm_enable_pin/snd_soc_dapm_disable_pin + 
snd_soc_dapm_sync.
This is the place where it is crashing, since dapm_sync calls dapm_power_widgets 
directly without locking.
All other places, when dapm_power_widgets called are protected by codec->mutex 
so those paths are safe.
By replacing the custom kcontrol/callbacks with SOC_DAPM_PIN_SWITCH() the 
problem is gone.
So it seams that my case can be solved easily with the SOC_DAPM_PIN_SWITCH(), 
but the race remains, and other setups might fail under some rare scenario

We can not add the codec->mutex to the snd_soc_dapm_sync, since it is called 
from soc_probe_dai_link, which is part of the initialization, and the codec-
>mutex has been taken up in the chain.

It might be a good idea to go through the machine drivers, and check for 
snd_soc_dapm_sync usage.

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