linux-kernel - Re: [PATCH] rcu: init and destroy rcu

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Date:   Wed, 15 Apr 2020 17:24:00 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Wei Yang <richard.weiyang@...il.com>
Cc:     josh@...htriplett.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: init and destroy rcu_synchronize when necessary

On Wed, Apr 15, 2020 at 10:26:55PM +0000, Wei Yang wrote:
> We would skip the rcu_synchronize if it is a duplicate call back function.
> 
> This is not necessary to init and destroy for them.
> 
> Signed-off-by: Wei Yang <richard.weiyang@...il.com>

Applied, thank you!

I edited the commit log a bit.  Could you please check below to make
sure that I didn't mess anything up?

							Thanx, Paul

------------------------------------------------------------------------

commit d9eaddf545fe8e3e2725e2fa0bf87b59b5667c14
Author: Wei Yang <richard.weiyang@...il.com>
Date:   Wed Apr 15 22:26:55 2020 +0000

    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@...il.com>
    Signed-off-by: Paul E. McKenney <paulmck@...nel.org>

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 3ce63a9..351c322 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -391,13 +391,14 @@ 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. */
@@ -408,9 +409,10 @@ 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);