[PATCH] rcu: Safe access to rcu_node structure's->exp_tasks

[Zqiang]

For kernels built with CONFIG_PREEMPT_RCU=y, the following scenario
can result system oops.

           CPU1                                           CPU2
rcu_preempt_deferred_qs_irqrestore                rcu_print_task_exp_stall
  if (special.b.blocked)                            READ_ONCE(rnp->exp_tasks) != NULL
    raw_spin_lock_rcu_node
    np = rcu_next_node_entry(t, rnp)
    if (&t->rcu_node_entry == rnp->exp_tasks)
      WRITE_ONCE(rnp->exp_tasks, np)
      ....
      raw_spin_unlock_irqrestore_rcu_node
                                                    raw_spin_lock_irqsave_rcu_node
                                                    t = list_entry(rnp->exp_tasks->prev,
                                                        struct task_struct, rcu_node_entry)
                                                    (if rnp->exp_tasks is NULL
                                                       will trigger oops)

This problem is that CPU2 accesses rcu_node structure's->exp_tasks
without holding the rcu_node structure's ->lock and CPU2 did not
observe CPU1's change to rcu_node structure's->exp_tasks in time,
if rcu_node structure's->exp_tasks is set null pointer by CPU1, after
that CPU2 accesses members of rcu_node structure's->exp_tasks will
trigger oops.

This commit therefore allows rcu_node structure's->exp_tasks to be
accessed while holding rcu_node structure's ->lock.

Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
---
 kernel/rcu/tree_exp.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 7cc4856da081..902e7c8709c7 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -803,9 +803,11 @@ static int rcu_print_task_exp_stall(struct rcu_node *rnp)
 	int ndetected = 0;
 	struct task_struct *t;
 
-	if (!READ_ONCE(rnp->exp_tasks))
-		return 0;
 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
+	if (!READ_ONCE(rnp->exp_tasks)) {
+		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+		return 0;
+	}
 	t = list_entry(rnp->exp_tasks->prev,
 		       struct task_struct, rcu_node_entry);
 	list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
-- 
2.25.1