Re: [RFC PATCH v3 05/13] ASoC: soc-pcm: align BE 'atomicity' with that of the FE

[Takashi Iwai <tiwai@suse.de>]

On Fri, 15 Oct 2021 18:22:58 +0200,
Pierre-Louis Bossart wrote:
> 
> 
> > The FE stream locks are necessary only two points: at adding and
> > deleting the BE in the link.  We used dpcm_lock in other places, but
> > those are superfluous or would make problem if converted to a FE
> > stream lock.
> 
> I must be missing a fundamental concept here - possibly a set of concepts...
> 
> It is my understanding that the FE-BE connection can be updated
> dynamically without any relationship to the usual ALSA steps, e.g. as a
> result of a control being changed by a user.
> 
> So if you only protect the addition/removal, isn't there a case where
> the for_each_dpcm_be() loop would either miss a BE or point to an
> invalid one?

No, for sleepable context, pcm_mutex is *always* taken when
adding/deleting a BE, and that's the main protection for the link.
The BE stream lock is taken additionally over it at adding/deleting a
BE, just for the code path via FE and BE trigger.

> In other words, don't we need the *same* lock to be used
> a) before changing and
> b) walking through the list?

> I also don't get what would happen if the dpcm_lock was converted to an
> FE stream lock. It works fine in my tests, so if there's limitation I
> didn't see it.

dpcm_lock was put in the places that could be recursively taken.
So this caused some deadlock, I suppose.

> >>> In addition, a lock around dpcm_show_state() might be needed to be
> >>> replaced with card->pcm_mutex, and we may need to revisit whether all
> >>> other paths take card->pcm_mutex.
> >>
> >> What happens if we show the state while a trigger happens? That's my
> >> main concern with using two separate locks (pcm_mutex and FE stream
> >> lock) to protect the same list, there are still windows of time where
> >> the list is not protected.
> > 
> > With the proper use of mutex, the list itself is protected.
> > If we need to protect the concurrent access to each BE in the show
> > method, an additional BE lock is needed in that part.  But that's a
> > subtle issue, as the link traversal itself is protected by the mutex.
> 
> If I look at your patch2, dpcm_be_disconnect() protects the list removal
> with the fe stream lock, but the show state is protected by both the
> pcm_mutex and the fe stream lock.

No, show_state() itself doesn't take any lock, but its caller
dpcm_state_read_file() takes the pcm_mutex.  That protects the list
addition / deletion.

> I have not been able to figure out when you need
> a) the pcm_mutex only
> b) the fe stream lock only
> c) both pcm_mutex and fe stream lock

The pcm_mutex is needed for every sleepable function that treat DPCM
FE link, but the mutex is taken only at the upper level, i.e. the
top-most caller like PCM ops FE itself or the DAPM calls.

That said, pcm_mutex is the top-most protection of BE links in FE.
But, there is a code path where a mutex can't be used, and that's the
FE and BE trigger.  For protecting against this, the FE stream lock is
taken only at the placing both adding and deleting a BE *in addition*.
At those places, both pcm_mutex and FE stream lock are taken.

BE stream lock is taken in addition below the above mutex and FE
locks.


Takashi