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Message-ID: <PH0PR11MB58801E84B4601F207B02377FDAFB9@PH0PR11MB5880.namprd11.prod.outlook.com>
Date: Fri, 6 Jan 2023 03:10:12 +0000
From: "Zhang, Qiang1" <qiang1.zhang@...el.com>
To: "Zhang, Qiang1" <qiang1.zhang@...el.com>,
"paulmck@...nel.org" <paulmck@...nel.org>,
"frederic@...nel.org" <frederic@...nel.org>,
"quic_neeraju@...cinc.com" <quic_neeraju@...cinc.com>,
"joel@...lfernandes.org" <joel@...lfernandes.org>
CC: "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] rcu: Safe access to rcu_node structure's->exp_tasks
>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@...el.com>
>
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
>
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