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

Joel Fernandes (Google)
2020-04-20T15:39:10Z

From: Sebastian Andrzej Siewior <bigeasy@linutronix.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@joelfernandes.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.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