Userspace RCU

This message on the lttng-dev mailing list discusses the reason why liburcu's default call_rcu worker thread data structures are leaked at process exit, and hints at a possible solution:

https://lists.lttng.org/pipermail/lttng-dev/2021-April/029951.html

Relevant snippet here:

I can see that maybe we could change liburcu to make it so that we free all
call_rcu data structures _if_ they happen to be empty of callbacks at process exit,
after invoking one rcu_barrier. That should take care of not leaking data structures
in the common case where call_rcu does not enqueue further callbacks.

And relevant follow up reply:

https://lists.lttng.org/pipermail/lttng-dev/2021-May/029963.html

Thanks for the response Mathieu. I believe that what was suggested in this discussion below would cover the current case that I'm seeing.

> > > I can see that maybe we could change liburcu to make it so that
> > > we
> > > free all
> > > call_rcu data structures _if_ they happen to be empty of
> > > callbacks at
> > > process exit,
> > > after invoking one rcu_barrier. That should take care of not
> > > leaking
> > > data structures
> > > in the common case where call_rcu does not enqueue further
> > > callbacks.
> > > 
> > > Thoughts ?

Without diving into the current urcu implementation more I don't have any better suggestions to handle the callback-in-callback issue that was mentioned.