Remove system calls from Jack callbacks

[theozanellato]

I am using libremidi for my audio application using Jack on Linux-RT. I create a Jack audio client, then pass it to libremidi as shown in the jack_share.cpp example. However performance is very poor due to the thread_lock semaphore syscalls that happen every block.

I am not sure what the best approach is for closing/unregistering the port safely. Here is a patch I am using for now to remove the syscalls.

libremidi-lock-patch.patch

[jcelerier]

Hello!
Did you try this under varied configuration ? I remember struggling and always ending up with various edge cases when trying with jack, pipewire under all sorts of memory & thread sanitizers.

Also, I think you don't need to add "this" to the lambda, it's already the &self

[theozanellato]

No I did not test this code very much at all, it is only there as an example. On second thought I don't think we should call jack's unregister function from the callback.

In our application we do not dynamically open/close new MIDI ports so I don't know what the best solution is for this use case. Maybe deactivate the client before and reactivate after closing ?
I know however that syscalls in the audio callback is a big no-no, so we have to patch libremidi in order to use it.