This Spin Lock is defined globally, This causes that my high priority
process to get blocked because of a lower priority process that holds
this spin lock. Of course you get priority inheritance, but this adds
quite a lot of extra execution time to the read function (because of
additional spin lock behavior, and scheduling to the lower priority
process)

It's a spin lock and its only held across very small numbers of
instructions in any normal path so this rather surprises me - in your
actual capture data can you see what is holding the lock for long times
causing this ?

Since the "user" and "refcount" reference counters have combined
behavior (refcount is decremented when users reaches zero), I don't
see a way to remove this global lock. Any ideas how this can be
improved?

I've never really thought about it because it should never be contended
in any meaningful way.

Alan