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

[Frederic Weisbecker <frederic@kernel.org>]

Le Thu, Sep 11, 2025 at 11:14:06AM -0400, Phil Auld a écrit :
> On Thu, Sep 11, 2025 at 05:02:45PM +0200 Frederic Weisbecker wrote:
> > 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.
> >
> 
> It does trigger pretty easily. We've done some work to try to make better
> (spreading HK work around for example) but you can still hit it. Especially,
> if there are virtualization layers involved...
> 
> Increasing that time a bit would be great :)

Interested in sending the patch? :-)

Thanks.

> 
> Cheers,
> Phil
> 
> 
> > Thanks.
> > 
> > -- 
> > Frederic Weisbecker
> > SUSE Labs
> > 
> 
> -- 
> 

-- 
Frederic Weisbecker
SUSE Labs