How the rdp->cblist enabled state is treated at CPU-hotplug time depends
on whether or not that ->cblist is offloaded.

1) Not offloaded: The ->cblist is disabled when the CPU goes down. All
   its callbacks are migrated and none can to enqueued until after some
   later CPU-hotplug operation brings the CPU back up.

2) Offloaded: The ->cblist is not disabled on CPU down because the CB/GP
   kthreads must finish invoking the remaining callbacks. There is thus
   no need to re-enable it on CPU up.

Since the ->cblist offloaded state is set in stone at boot, it cannot
change between CPU down and CPU up. So 1) and 2) are symmetrical.

However, given runtime toggling of the offloaded state, there are two
additional asymmetrical scenarios:

3) The ->cblist is not offloaded when the CPU goes down. The ->cblist
   is later toggled to offloaded and then the CPU comes back up.

4) The ->cblist is offloaded when the CPU goes down. The ->cblist is
   later toggled to no longer be offloaded and then the CPU comes back up.

Scenario 4) is currently handled correctly. The ->cblist remains enabled
on CPU down and gets re-initialized on CPU up. The toggling operation
will wait until ->cblist is empty, so ->cblist will remain empty until
CPU-up time.

The scenario 3) would run into trouble though, as the rdp is disabled
on CPU down and not re-initialized/re-enabled on CPU up.  Except that
in this case, ->cblist is guaranteed to be empty because all its
callbacks were migrated away at CPU-down time.  And the CPU-up code
already initializes and enables any empty ->cblist structures in order
to handle the possibility of early-boot invocations of call_rcu() in
the case where such invocations don't occur.  So all that need be done
is to adjust the locking.

Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Inspired-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>