rcu: Use rsp->expedited_wq instead of sync_rcu_preempt_exp_wq

Now that there is an ->expedited_wq waitqueue in each rcu_state structure,
there is no need for the sync_rcu_preempt_exp_wq global variable.  This
commit therefore substitutes ->expedited_wq for sync_rcu_preempt_exp_wq.
It also initializes ->expedited_wq only once at boot instead of at the
start of each expedited grace period.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -3556,7 +3556,6 @@ void synchronize_sched_expedited(void)
 	rcu_exp_gp_seq_start(rsp);
 
 	/* Stop each CPU that is online, non-idle, and not us. */
-	init_waitqueue_head(&rsp->expedited_wq);
 	atomic_set(&rsp->expedited_need_qs, 1); /* Extra count avoids race. */
 	for_each_online_cpu(cpu) {
 		struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
@@ -4179,6 +4178,7 @@ static void __init rcu_init_one(struct rcu_state *rsp,
 	}
 
 	init_waitqueue_head(&rsp->gp_wq);
+	init_waitqueue_head(&rsp->expedited_wq);
 	rnp = rsp->level[rcu_num_lvls - 1];
 	for_each_possible_cpu(i) {
 		while (i > rnp->grphi)

kernel/rcu/tree_plugin.h

@@ -535,8 +535,6 @@ void synchronize_rcu(void)
 }
 EXPORT_SYMBOL_GPL(synchronize_rcu);
 
-static DECLARE_WAIT_QUEUE_HEAD(sync_rcu_preempt_exp_wq);
-
 /*
  * Return non-zero if there are any tasks in RCU read-side critical
  * sections blocking the current preemptible-RCU expedited grace period.
@@ -590,7 +588,7 @@ static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp,
 			raw_spin_unlock_irqrestore(&rnp->lock, flags);
 			if (wake) {
 				smp_mb(); /* EGP done before wake_up(). */
-				wake_up(&sync_rcu_preempt_exp_wq);
+				wake_up(&rsp->expedited_wq);
 			}
 			break;
 		}
@@ -729,7 +727,7 @@ void synchronize_rcu_expedited(void)
 
 	/* Wait for snapshotted ->blkd_tasks lists to drain. */
 	rnp = rcu_get_root(rsp);
-	wait_event(sync_rcu_preempt_exp_wq,
+	wait_event(rsp->expedited_wq,
 		   sync_rcu_preempt_exp_done(rnp));
 
 	/* Clean up and exit. */