If an unloadable module uses RCU callbacks, it need to use
rcu_barrier() so that the module may be safely unloaded.
While hacking on a netfilter module of my own, I learned the
importance of calling rcu_barrier() instead of only a
synchronize_rcu() on module unload (iif using any call_rcu()
callbacks). synchronize_rcu() does wait for a grace period to
elapse, but it does not wait for the callbacks to complete.
...
I have made a patch for each individual module, so objections can be
made on a per module basis. I have Cc'ed all of the patches to the
maintainers of each module (according to the MAINTAINERS file).