Re: [PATCH 3/4] workqueue: Tag bound workers with KTHREAD_IS_PER_CPU

[Lai Jiangshan]

On Sat, Jan 16, 2021 at 11:16 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Sat, Jan 16, 2021 at 10:45:04PM +0800, Lai Jiangshan wrote:
> > On Sat, Jan 16, 2021 at 8:45 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > > It is also the exact sequence normal per-cpu threads (smpboot) use to
> > > preserve affinity.
> >
> > Other per-cpu threads normally do short-live works. wq's work can be
> > lengthy, cpu-intensive, heavy-lock-acquiring or even call
> > get_online_cpus() which might result in a deadlock with kthread_park().
>
> kthread_park() is called by the migration thread running the
> workqueue_online_cpu() callback.
>
> kthread_parkme() is called by the worker thread, after it completes a
> work and has no locks held from that context.
>
>

BP:                 AP:                  worker:
cpus_write_lock()
bringup_cpu()                            work_item_func()
  bringup_wait_for_ap                      get_online_cpus()
                    kthread_park(worker)