> 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? 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. >>> 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. 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
