Merge branch 'transfer-rcu-lock-state-across-subprog-calls'

Merge branch 'transfer-rcu-lock-state-across-subprog-calls'

Kumar Kartikeya Dwivedi says:

====================
Transfer RCU lock state across subprog calls

David suggested during the discussion in [0] that we should handle RCU
locks in a similar fashion to spin locks where the verifier understands
when a lock held in a caller is released in callee, or lock taken in
callee is released in a caller, or the callee is called within a lock
critical section. This set extends the same semantics to RCU read locks
and adds a few selftests to verify correct behavior. This issue has also
come up for sched-ext programs.

This would now allow static subprog calls to be made without errors
within RCU read sections, for subprogs to release RCU locks of callers
and return to them, or for subprogs to take RCU lock which is later
released in the caller.

  [0]: https://lore.kernel.org/bpf/20240204120206.796412-1-memxor@gmail.com

Changelog:
----------
v1 -> v2:
v1: https://lore.kernel.org/bpf/20240204230231.1013964-1-memxor@gmail.com

 * Add tests for global subprog behaviour (Yafang)
 * Add Acks, Tested-by (Yonghong, Yafang)
====================

Link: https://lore.kernel.org/r/20240205055646.1112186-1-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>