[tip: core/rcu] rcu: Initialize and destroy rcu_synchronize only when necessary

[tip-bot2 for Wei Yang]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     7ee880b7bf1dea88d0a472b775aebdb4fb6bf860
Gitweb:        https://git.kernel.org/tip/7ee880b7bf1dea88d0a472b775aebdb4fb6bf860
Author:        Wei Yang <richard.weiyang@xxxxxxxxx>
AuthorDate:    Wed, 15 Apr 2020 22:26:55 
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Mon, 29 Jun 2020 11:58:49 -07:00

rcu: Initialize and destroy rcu_synchronize only when necessary

The __wait_rcu_gp() function unconditionally initializes and cleans up
each element of rs_array[], whether used or not.  This is slightly
wasteful and rather confusing, so this commit skips both initialization
and cleanup for duplicate callback functions.

Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 kernel/rcu/update.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 84843ad..f5a82e1 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -390,13 +390,14 @@ void __wait_rcu_gp(bool checktiny, int n, call_rcu_func_t *crcu_array,
 			might_sleep();
 			continue;
 		}
-		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)
+		if (j == i) {
+			init_rcu_head_on_stack(&rs_array[i].head);
+			init_completion(&rs_array[i].completion);
 			(crcu_array[i])(&rs_array[i].head, wakeme_after_rcu);
+		}
 	}
 
 	/* Wait for all callbacks to be invoked. */
@@ -407,9 +408,10 @@ void __wait_rcu_gp(bool checktiny, int n, call_rcu_func_t *crcu_array,
 		for (j = 0; j < i; j++)
 			if (crcu_array[j] == crcu_array[i])
 				break;
-		if (j == i)
+		if (j == i) {
 			wait_for_completion(&rs_array[i].completion);
-		destroy_rcu_head_on_stack(&rs_array[i].head);
+			destroy_rcu_head_on_stack(&rs_array[i].head);
+		}
 	}
 }
 EXPORT_SYMBOL_GPL(__wait_rcu_gp);