Linux docs explicitly state that this is not required :
"Important note: The rcu_barrier() function is not, repeat, not,
obligated to wait for a grace period. It is instead only required to
wait for RCU callbacks that have already been posted. Therefore, if
there are no RCU callbacks posted anywhere in the system, rcu_barrier()
is within its rights to return immediately. Even if there are
callbacks posted, rcu_barrier() does not necessarily need to wait for
a grace period."
P.S. This makes my current workaround for i915 double-free related
crashes on drm-kmod v5.5 cheaper.