Re: [PATCH] sched/core: Fix potential deadlock on rq lock

[Peter Zijlstra]

On Thu, Sep 11, 2025 at 12:42:49PM +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:
> 
> CPU0								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@xxxxxxxxxx>

I fundamentally hate that deferred thing and consider it a printk bug.

But really, if you trip that WARN, fix it and the problem goes away.