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

[Zhang, Qiang1]

>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>
>


Hi Paul,  some suggestions for this modification?

Thanks
Zqiang



>---
> 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
>