rcu: Remove redundant READ_ONCE() of rcu_state.gp_flags in tree.c

Although it is functionally OK to do READ_ONCE() of a variable that
cannot change, it is confusing and at best an accident waiting to happen.
This commit therefore removes a number of READ_ONCE(rcu_state.gp_flags)
instances from kernel/rcu/tree.c that are not needed due to updates
to this field being excluded by virtue of holding the root rcu_node
structure's ->lock.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Closes: https://lore.kernel.org/lkml/4857c5ef-bd8f-4670-87ac-0600a1699d05@paulmck-laptop/T/#mccb23c2a4902da4d3c750165329f8de056903c58
Reported-by: Julia Lawall <julia.lawall@inria.fr>
Closes: https://lore.kernel.org/lkml/4857c5ef-bd8f-4670-87ac-0600a1699d05@paulmck-laptop/T/#md1b5c026584f9c3c7b0fbc9240dd7de584597b73
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

Author: paulmckrcu

kernel/rcu/tree.c

@@ -1463,7 +1463,7 @@ static noinline_for_stack bool rcu_gp_init(void)
 
 	WRITE_ONCE(rcu_state.gp_activity, jiffies);
 	raw_spin_lock_irq_rcu_node(rnp);
-	if (!READ_ONCE(rcu_state.gp_flags)) {
+	if (!rcu_state.gp_flags) {
 		/* Spurious wakeup, tell caller to go back to sleep.  */
 		raw_spin_unlock_irq_rcu_node(rnp);
 		return false;
@@ -1648,8 +1648,7 @@ static void rcu_gp_fqs(bool first_time)
 	/* Clear flag to prevent immediate re-entry. */
 	if (READ_ONCE(rcu_state.gp_flags) & RCU_GP_FLAG_FQS) {
 		raw_spin_lock_irq_rcu_node(rnp);
-		WRITE_ONCE(rcu_state.gp_flags,
-			   READ_ONCE(rcu_state.gp_flags) & ~RCU_GP_FLAG_FQS);
+		WRITE_ONCE(rcu_state.gp_flags, rcu_state.gp_flags & ~RCU_GP_FLAG_FQS);
 		raw_spin_unlock_irq_rcu_node(rnp);
 	}
 }
@@ -1910,8 +1909,7 @@ static void rcu_report_qs_rsp(unsigned long flags)
 {
 	raw_lockdep_assert_held_rcu_node(rcu_get_root());
 	WARN_ON_ONCE(!rcu_gp_in_progress());
-	WRITE_ONCE(rcu_state.gp_flags,
-		   READ_ONCE(rcu_state.gp_flags) | RCU_GP_FLAG_FQS);
+	WRITE_ONCE(rcu_state.gp_flags, rcu_state.gp_flags | RCU_GP_FLAG_FQS);
 	raw_spin_unlock_irqrestore_rcu_node(rcu_get_root(), flags);
 	rcu_gp_kthread_wake();
 }
@@ -2426,8 +2424,7 @@ void rcu_force_quiescent_state(void)
 		raw_spin_unlock_irqrestore_rcu_node(rnp_old, flags);
 		return;  /* Someone beat us to it. */
 	}
-	WRITE_ONCE(rcu_state.gp_flags,
-		   READ_ONCE(rcu_state.gp_flags) | RCU_GP_FLAG_FQS);
+	WRITE_ONCE(rcu_state.gp_flags, rcu_state.gp_flags | RCU_GP_FLAG_FQS);
 	raw_spin_unlock_irqrestore_rcu_node(rnp_old, flags);
 	rcu_gp_kthread_wake();
 }