[RFC PATCH v2 0/5] ASoC: soc-pcm: fix trigger race conditions with shared BE

[Pierre-Louis Bossart]

>> dpcm_be_disconnect already uses a spin_lock around
>>
>> list_del(&dpcm->list_be);
>> list_del(&dpcm->list_fe);
>>
>> and in some other places, are you suggesting we change those to the FE lock?
> 
> Basically yes.
> 
>> Otherwise, I understood your proposal as using three locks (existing
>> spinlock, FE lock, BE lock) to deal with DPCM. If the existing spinlock
>> and FE lock are combined, we'd still have two locks.
> 
> Stream locks are more fine-grained, hence more efficient :)
> The card-level spinlock is superfluous and it can go away.
> 
>> I was suggesting we use only one ;-)
> 
> Basically we need to protect two things:
> - The BE links
> - The concurrent accesses to BEs
> The former belongs to each FE that holds the links, and the FE stream
> lock would cover.  The latter is rather a per-BE business.
> 
> An oft-seen risk of multiple locks is deadlocking, but in this case,
> as long as we keep the lock order FE->BE, nothing wrong can happen.

famous last words  "nothing wrong can happen." :-)

I already added a helper to do this FE lock, I can easily replace the
implementation to remove the spin_lock and use the FE PCM lock.
we might even add the lock in the definition of for_each_dpcm_be() to
avoid misses.

Let me try this out today, thanks for the suggestions.