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

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Sep 11, 2025 at 08:30:44PM +0800, wangtao (EQ) wrote:
> 
> 在 2025/9/11 20:20, Greg KH 写道:
> > 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@huawei.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
> > > 
> > > 
> Sorry, we initially discovered the issue while testing the stable branch,
> and it seems that the mainline has the same problem, but I haven't submitted
> a patch yet.

That is required for us to be able to take a patch into the stable
trees, thanks.

greg k-h