rcu: Prevent initialization-time quiescent-state race

rcu: Prevent initialization-time quiescent-state race

Now the the grace-period initialization procedure is preemptible, it is
subject to the following race on systems whose rcu_node tree contains
more than one node:

1.	CPU 31 starts initializing the grace period, including the
	first leaf rcu_node structures, and is then preempted.

2.	CPU 0 refers to the first leaf rcu_node structure, and notes
	that a new grace period has started.  It passes through a
	quiescent state shortly thereafter, and informs the RCU core
	of this rite of passage.

3.	CPU 0 enters an RCU read-side critical section, acquiring
	a pointer to an RCU-protected data item.

4.	CPU 31 removes the data item referenced by CPU 0 from the
	data structure, and registers an RCU callback in order to
	free it.

5.	CPU 31 resumes initializing the grace period, including its
	own rcu_node structure.  In invokes rcu_start_gp_per_cpu(),
	which advances all callbacks, including the one registered
	in #4 above, to be handled by the current grace period.

6.	The remaining CPUs pass through quiescent states and inform
	the RCU core, but CPU 0 remains in its RCU read-side critical
	section, still referencing the now-removed data item.

7.	The grace period completes and all the callbacks are invoked,
	including the one that frees the data item that CPU 0 is still
	referencing.  Oops!!!

This commit therefore moves the callback handling to precede initialization
of any of the rcu_node structures, thus avoiding this race.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>