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Currently, rcu_barrier_tasks(), rcu_barrier_tasks_rude(),
and rcu_barrier_tasks_trace() simply invoke the corresponding
synchronize_rcu_tasks*() function.  This works because there is only
one callback queue.

However, there will soon be multiple callback queues.  This commit
therefore scans the queues currently in use, entraining a callback on
each non-empty queue.  Sequence numbers and reference counts are used
to synchronize this process in a manner similar to the approach taken
by rcu_barrier().
Reported-by: Martin Lau <kafai@fb.com>
Cc: Neeraj Upadhyay <neeraj.iitr10@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>