[PATCH] rcu: Remove unnecessary READ_ONCE in rcu_preempt_read_enter/exit

[Alan Huang <mmpgouride@gmail.com>]

The two READ_ONCEs here imply that there might be updater concurrently
modifies rcu_read_lock_nesting, although it's true there could be
interrupt handlers enter rcu RSCS, but rcu_read_lock_nesting should
remain unchanged after they finished. That is, READ_ONCE and then
WRITE_ONCE the same variable is not a valid pattern, this patch thus
removes the two READ_ONCEs.

Signed-off-by: Alan Huang <mmpgouride@gmail.com>
---
 kernel/rcu/tree_plugin.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 340bbefe5f65..c90edbb7fa40 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -376,12 +376,12 @@ static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
 
 static void rcu_preempt_read_enter(void)
 {
-	WRITE_ONCE(current->rcu_read_lock_nesting, READ_ONCE(current->rcu_read_lock_nesting) + 1);
+	WRITE_ONCE(current->rcu_read_lock_nesting, current->rcu_read_lock_nesting + 1);
 }
 
 static int rcu_preempt_read_exit(void)
 {
-	int ret = READ_ONCE(current->rcu_read_lock_nesting) - 1;
+	int ret = current->rcu_read_lock_nesting - 1;
 
 	WRITE_ONCE(current->rcu_read_lock_nesting, ret);
 	return ret;
-- 
2.34.1