rcu: Use READ_ONCE() for ->expmask in rcu_read_unlock_special()

The rcu_node structure's ->expmask field is updated only when holding the
->lock, but is also accessed locklessly.  This means that all ->expmask
updates must use WRITE_ONCE() and all reads carried out without holding
->lock must use READ_ONCE().  This commit therefore changes the lockless
->expmask read in rcu_read_unlock_special() to use READ_ONCE().

Reported-by: syzbot+99f4ddade3c22ab0cf23@syzkaller.appspotmail.com
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Marco Elver <elver@google.com>

kernel/rcu/tree_plugin.h

@@ -599,7 +599,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
 		struct rcu_node *rnp = rdp->mynode;
 
 		exp = (t->rcu_blocked_node && t->rcu_blocked_node->exp_tasks) ||
-		      (rdp->grpmask & rnp->expmask) ||
+		      (rdp->grpmask & READ_ONCE(rnp->expmask)) ||
 		      tick_nohz_full_cpu(rdp->cpu);
 		// Need to defer quiescent state until everything is enabled.
 		if (irqs_were_disabled && use_softirq &&