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

[Frederic Weisbecker <frederic@kernel.org>]

Le Thu, Sep 11, 2025 at 03:53:58PM +0200, Peter Zijlstra a écrit :
> 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@huawei.com>
> 
> 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.

And probably it triggers a lot of false positives. An overloaded housekeeping
CPU can easily be off for 2 seconds. We should make it 30 seconds.

Thanks.

-- 
Frederic Weisbecker
SUSE Labs