linux-kernel - Re: [PATCH stable/linux-5.10.y] sched/core: Fix potential deadlock on rq lock

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Message-ID: <2025091123-unsterile-why-ca1e@gregkh>
Date: Thu, 11 Sep 2025 14:20:35 +0200
From: Greg KH <gregkh@...uxfoundation.org>
To: Wang Tao <wangtao554@...wei.com>
Cc: stable@...r.kernel.org, mingo@...hat.com, peterz@...radead.org,
	juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, bristot@...hat.com, tglx@...utronix.de,
	frederic@...nel.org, linux-kernel@...r.kernel.org,
	tanghui20@...wei.com, zhangqiao22@...wei.com
Subject: Re: [PATCH stable/linux-5.10.y] sched/core: Fix potential deadlock
 on rq lock

On Mon, Sep 08, 2025 at 08:42:30AM +0000, Wang Tao wrote:
> When CPU 1 enters the nohz_full state, and the kworker on CPU 0 executes
> the function sched_tick_remote, holding the lock on CPU1's rq
> and triggering the warning WARN_ON_ONCE(delta > (u64)NSEC_PER_SEC * 3).
> This leads to the process of printing the warning message, where the
> console_sem semaphore is held. At this point, the print task on the
> CPU1's rq cannot acquire the console_sem and joins the wait queue,
> entering the UNINTERRUPTIBLE state. It waits for the console_sem to be
> released and then wakes up. After the task on CPU 0 releases
> the console_sem, it wakes up the waiting console_sem task.
> In try_to_wake_up, it attempts to acquire the lock on CPU1's rq again,
> resulting in a deadlock.
> 
> The triggering scenario is as follows:
> 
> CPU 0								CPU1
> sched_tick_remote
> WARN_ON_ONCE(delta > (u64)NSEC_PER_SEC * 3)
> 
> report_bug							con_write
> printk
> 
> console_unlock
> 								do_con_write
> 								console_lock
> 								down(&console_sem)
> 								list_add_tail(&waiter.list, &sem->wait_list);
> up(&console_sem)
> wake_up_q(&wake_q)
> try_to_wake_up
> __task_rq_lock
> _raw_spin_lock
> 
> This patch fixes the issue by deffering all printk console printing
> during the lock holding period.
> 
> Fixes: d84b31313ef8 ("sched/isolation: Offload residual 1Hz scheduler tick")
> Signed-off-by: Wang Tao <wangtao554@...wei.com>
> ---
>  kernel/sched/core.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 40f40f359c5d..fd2c83058ec2 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4091,6 +4091,7 @@ static void sched_tick_remote(struct work_struct *work)
>  		goto out_requeue;
>  
>  	rq_lock_irq(rq, &rf);
> +	printk_deferred_enter();
>  	curr = rq->curr;
>  	if (cpu_is_offline(cpu))
>  		goto out_unlock;
> @@ -4109,6 +4110,7 @@ static void sched_tick_remote(struct work_struct *work)
>  
>  	calc_load_nohz_remote(rq);
>  out_unlock:
> +	printk_deferred_exit();
>  	rq_unlock_irq(rq, &rf);
>  out_requeue:
>  
> -- 
> 2.34.1
> 
> 

What is the git commit id of this in Linus's tree?

thanks,

greg k-h