Re: [PATCH 1/1] kthread: break dependency between worker->lock and task_struct->pi_lock

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Apr 27, 2020 at 11:43:58AM -0700, Suren Baghdasaryan wrote:
> A number of kthread-related functions indirectly take task_struct->pi_lock
> while holding worker->lock in the call chain like this:
>     spin_lock(&worker->lock)
>     kthread_insert_work
>     wake_up_process
>     try_to_wake_up
>     raw_spin_lock_irqsave(&p->pi_lock, flags)
> 
> This lock dependency exists whenever kthread_insert_work is called either
> directly or indirectly via __kthread_queue_delayed_work in the following
> functions:
>     kthread_queue_work
>     kthread_delayed_work_timer_fn
>     kthread_queue_delayed_work
>     kthread_flush_work
>     kthread_mod_delayed_work
> 
> This creates possibilities for circular dependencies like the one reported
> at: https://lkml.org/lkml/2020/4/24/954

Please, do not use lkml.org links.

Also, ideally, we'd pull that kthread_queue_delayed_work() out from
under rq->lock.

In fact, looking at it, WTH is the delayed branch of
kthread_queue_delayed_work() under that lock? That whole
delayed_work_list thing smells like bong-hits.