rcu: Make synchronize_rcu_mult() check for duplicates

Currently, doing synchronize_rcu_mult(call_rcu, call_rcu) might
(or might not) wait for two RCU grace periods.  One approach is
of course "don't do that!", but in CONFIG_PREEMPT=n kernels,
synchronize_rcu_mult(call_rcu, call_rcu_sched) does exactly that.
This results in an ugly #ifdef in sched_cpu_deactivate().

This commit therefore makes __wait_rcu_gp() check for duplicates,
which in turn allows duplicates to be passed to synchronize_rcu_mult()
without risk of waiting twice on the same type of grace period.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/update.c

@@ -379,6 +379,7 @@ void __wait_rcu_gp(bool checktiny, int n, call_rcu_func_t *crcu_array,
 		   struct rcu_synchronize *rs_array)
 {
 	int i;
+	int j;
 
 	/* Initialize and register callbacks for each flavor specified. */
 	for (i = 0; i < n; i++) {
@@ -390,6 +391,10 @@ void __wait_rcu_gp(bool checktiny, int n, call_rcu_func_t *crcu_array,
 		}
 		init_rcu_head_on_stack(&rs_array[i].head);
 		init_completion(&rs_array[i].completion);
+		for (j = 0; j < i; j++)
+			if (crcu_array[j] == crcu_array[i])
+				break;
+		if (j == i)
 			(crcu_array[i])(&rs_array[i].head, wakeme_after_rcu);
 	}
 
@@ -399,6 +404,10 @@ void __wait_rcu_gp(bool checktiny, int n, call_rcu_func_t *crcu_array,
 		    (crcu_array[i] == call_rcu ||
 		     crcu_array[i] == call_rcu_bh))
 			continue;
+		for (j = 0; j < i; j++)
+			if (crcu_array[j] == crcu_array[i])
+				break;
+		if (j == i)
 			wait_for_completion(&rs_array[i].completion);
 		destroy_rcu_head_on_stack(&rs_array[i].head);
 	}