linux-kernel - [PATCH rcu/dev -fixes 3/4] rcu/tree: Avoid using xchg() in kfree_call_rcu_add_ptr_to

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Message-Id: <20200420153837.194532-4-joel@joelfernandes.org>
Date:   Mon, 20 Apr 2020 11:38:36 -0400
From:   "Joel Fernandes (Google)" <joel@...lfernandes.org>
To:     linux-kernel@...r.kernel.org
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>, urezki@...il.com
Subject: [PATCH rcu/dev -fixes 3/4] rcu/tree: Avoid using xchg() in kfree_call_rcu_add_ptr_to_bulk()

From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

There is no need to use xchg(), the access is serialized by krcp->lock.
The xchg() function adds some atomic barriers which remain hidden in
x86's disassembly but are visible on ARM for instance.

Replace xchg() with a load + store.

Acked-by: Joel Fernandes (Google) <joel@...lfernandes.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 kernel/rcu/tree.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index cd61649e1b001..f6eb3aee0935e 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3088,7 +3088,8 @@ kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp,
 	/* Check if a new block is required. */
 	if (!krcp->bhead ||
 			krcp->bhead->nr_records == KFREE_BULK_MAX_ENTR) {
-		bnode = xchg(&krcp->bcached, NULL);
+		bnode = krcp->bcached;
+		krcp->bcached = NULL;
 		if (!bnode) {
 			WARN_ON_ONCE(sizeof(struct kfree_rcu_bulk_data) > PAGE_SIZE);
 
-- 
2.26.1.301.g55bc3eb7cb9-goog