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>

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index c3a32717c42d9fb3a4d9a2afa6e00012bc836398..698a7f195f8822d9200b1fdaf6363bbdccc48d05 100644 (file)
--- a/kernel/rcu/tree_plugin.h
+++ b/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 &&