[alsa-devel] [PATCH] ASoC: dapm: Fix locking during codec shutdown

[Liam Girdwood]

On Fri, 2012-07-06 at 17:43 +0100, Mark Brown wrote:
> On Fri, Jul 06, 2012 at 05:36:15PM +0100, Liam Girdwood wrote:
> 
> > I don't think there is anything explicitly stopping a power off from
> > happening during a stream shutdown atm without this patch. It was
> > reported to me that this would fail 1 in 20 times on a certain device
> > and that the above patch fixed the issue (1200 tests completed OK after
> > the patch).
> 
> > I do think we need this patch atm since we are accessing the same
> > resources as the stream event.
> 
> So how is suspend and resume safe, for example?  It's the same general
> path through the PM core.

suspend and resume are safe as they call stream_event() that holds the
DAPM mutex.

Fwiw, I've searched my email and found Misa's analysis here :-

<device is preparing to reboot>
soc_dapm_shutdown_codec() : Insert widget A, B, C, ... in "down_list"
soc_dapm_shutdown_codec() : Run power sequence via dapm_seq_run()
dapm_seq_run(): Move widget A, B, C, ... from "down_list" to "pending"
list for coalesced changes
dapm_seq_run_coalesced(): Iterating over A

<system sound indicating device reboot closes>
dapm_power_widgets() : Run a STREAM_STOP event a inserts widget B in a
different "down_list" (whose next element is not C, but other widget X
with diff reg address)

dapm_seq_run_coalesced() : Iterates over B and expected C next, but got
the widget X added by dapm_power_widgets() above
dapm_seq_run_coalesced() : Widget X doesn't share same reg address as
widget A, B, ... hence BUG().

dapm_power_widgets() uses card's power_mutex, but
soc_dapm_shutdown_codec() does not, so above scenario is possible. IMO,
just protecting codec shutdown with card's power_mutex should suffice as
in attached patch.

Liam