Re: [tim-janik/beast] BSE: SF2: use shared engine module instead of global soundfont lock (#85)

[Stefan Westerfeld <notifications github com>]

Recently, we discussed this merge request, and you (Tim) said you do not want to merge code that locks a mutex during audio processing due to priority inversion.

So, what does the lock protect here? There is one global fluid_synth instance that is used by both, the soundfont repo and the soundfont osc code. There is also a bunch of data shared between the soundfont osc modules that is protected by the lock. Locks are acquired

• by the soundfont repo when loading/unloading soundfonts
• by the soundfont osc modules when rendering audio

Like the wave repo code, we never load/unload soundfonts if the repo is prepared, so effectively the soundfont repo will not lock the global mutex while the project is playing.

The soundfont osc modules are

• one common module that does the actual rendering (1)
• per osc modules that just copy the data out, and maybe change the preset of the current voice (2)

But these modules are all executed within the engine process() callback so they effectively run with the highest priority we have. So there is no priority inversion possible here. Also note that while (1) takes a while because it renders the data, (2) is really cheap, because the audio data is already available.

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