srcu: Expedite SRCU-fast grace periods

Currently, SRCU-fast grace periods use synchronize_rcu() to provide the
needed ordering with readers, even given an expedited SRCU-fast grace
period, which isn't all that expedited.  This commit therefore instead
uses synchronize_rcu_expedited() if there is an expedited SRCU-fast
grace period in flight.

Of course, given an non-expedited SRCU-fast grace period blocked in
synchronize_rcu(), a later request for an expedited SRCU-fast grace
period will wait for that synchronize_rcu() to return before switching
to use of synchronize_rcu_expedited().  If this turns out to be a real
problem for a production workload, we can increase the complexity (but
likely also degrade the energy efficiency) to speed things up further.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org>

kernel/rcu/srcutree.c

@@ -502,6 +502,8 @@ static bool srcu_readers_active_idx_check(struct srcu_struct *ssp, int idx)
 	 */
 	if (!did_gp)
 		smp_mb(); /* A */
+	else if (srcu_gp_is_expedited(ssp))
+		synchronize_rcu_expedited(); /* X */
 	else
 		synchronize_rcu(); /* X */