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Subject: Re: [PATCH] rcu: fix a deadlock caused by not release rcu_node->lock
From:   "Xu, Yanfei" <yanfei.xu@windriver.com>
To:     paulmck@kernel.org, josh@joshtriplett.org, rostedt@goodmis.org,
        mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com,
        joel@joelfernandes.org
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org
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sorry, please ignore this patch...

Regards,
Yanfei

On 5/16/21 5:28 PM, yanfei.xu@windriver.com wrote:
> From: Yanfei Xu <yanfei.xu@windriver.com>
> 
> rcu_node->lock isn't released in rcu_print_task_stall() if the rcu_node
> don't contain tasks which blocking the GP. However this rcu_node->lock
> will be used again in rcu_dump_cpu_stacks() soon while the ndetected is
> non-zero. As a result the cpu will hung by this deadlock.
> 
> Signed-off-by: Yanfei Xu <yanfei.xu@windriver.com>
> ---
>   kernel/rcu/tree_stall.h | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> index b72311d24a9f..4e97c9977d1c 100644
> --- a/kernel/rcu/tree_stall.h
> +++ b/kernel/rcu/tree_stall.h
> @@ -267,8 +267,10 @@ static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
>   	struct task_struct *ts[8];
>   
>   	lockdep_assert_irqs_disabled();
> -	if (!rcu_preempt_blocked_readers_cgp(rnp))
> +	if (!rcu_preempt_blocked_readers_cgp(rnp)) {
> +		raw_spin_lock_irqsave_rcu_node(rnp, flags);
>   		return 0;
> +	}
>   	pr_err("\tTasks blocked on level-%d rcu_node (CPUs %d-%d):",
>   	       rnp->level, rnp->grplo, rnp->grphi);
>   	t = list_entry(rnp->gp_tasks->prev,
>