rcu: Remove READ_ONCE() for rdp->gpwrap access in __note_gp_changes()

There is one access to the per-CPU rdp->gpwrap field in the
__note_gp_changes() function that does not use READ_ONCE(), but all other
accesses do use READ_ONCE().  When using the 8*TREE03 and CONFIG_NR_CPUS=8
configuration, KCSAN found no data races at that point.  This is because
all calls to __note_gp_changes() hold rnp->lock, which excludes writes
to the rdp->gpwrap fields for all CPUs associated with that same leaf
rcu_node structure.

This commit therefore removes READ_ONCE() from rdp->gpwrap accesses
within the __note_gp_changes() function.

Signed-off-by: Zilin Guan <zilinguan811@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

Author: Zilin Guan

kernel/rcu/tree.c

@@ -1254,7 +1254,7 @@ static bool __note_gp_changes(struct rcu_node *rnp, struct rcu_data *rdp)
 
 	/* Handle the ends of any preceding grace periods first. */
 	if (rcu_seq_completed_gp(rdp->gp_seq, rnp->gp_seq) ||
-	    unlikely(READ_ONCE(rdp->gpwrap))) {
+	    unlikely(rdp->gpwrap)) {
 		if (!offloaded)
 			ret = rcu_advance_cbs(rnp, rdp); /* Advance CBs. */
 		rdp->core_needs_qs = false;
@@ -1268,7 +1268,7 @@ static bool __note_gp_changes(struct rcu_node *rnp, struct rcu_data *rdp)
 
 	/* Now handle the beginnings of any new-to-this-CPU grace periods. */
 	if (rcu_seq_new_gp(rdp->gp_seq, rnp->gp_seq) ||
-	    unlikely(READ_ONCE(rdp->gpwrap))) {
+	    unlikely(rdp->gpwrap)) {
 		/*
 		 * If the current grace period is waiting for this CPU,
 		 * set up to detect a quiescent state, otherwise don't
@@ -1283,7 +1283,7 @@ static bool __note_gp_changes(struct rcu_node *rnp, struct rcu_data *rdp)
 	rdp->gp_seq = rnp->gp_seq;  /* Remember new grace-period state. */
 	if (ULONG_CMP_LT(rdp->gp_seq_needed, rnp->gp_seq_needed) || rdp->gpwrap)
 		WRITE_ONCE(rdp->gp_seq_needed, rnp->gp_seq_needed);
-	if (IS_ENABLED(CONFIG_PROVE_RCU) && READ_ONCE(rdp->gpwrap))
+	if (IS_ENABLED(CONFIG_PROVE_RCU) && rdp->gpwrap)
 		WRITE_ONCE(rdp->last_sched_clock, jiffies);
 	WRITE_ONCE(rdp->gpwrap, false);
 	rcu_gpnum_ovf(rnp, rdp);