[PATCH 5.5 037/367] rcu: Use READ_ONCE() for ->expmask in rcu_read_unlock_special()

[Greg Kroah-Hartman]

From: Paul E. McKenney <paulmck@xxxxxxxxxx>

commit c51f83c315c392d9776c33eb16a2fe1349d65c7f upstream.

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@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Acked-by: Marco Elver <elver@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/rcu/tree_plugin.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -612,7 +612,7 @@ static void rcu_read_unlock_special(stru
 
 		t->rcu_read_unlock_special.b.exp_hint = false;
 		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 &&