[bug-report] task info hung problem in fb_deferred_io_work()

[Patrik Jakobsson]

On Fri, Apr 19, 2024 at 5:34 PM Nam Cao <namcao at linutronix.de> wrote:
>
> On 2024-04-19 Patrik Jakobsson wrote:
> > Neither cancel_delayed_work_sync() or flush_delayed_work() prevent new
> > work from being scheduled after they return.
>
> flush_delayed_work() is called during device closing. And because no
> writes are performed after the device has been closed, no new work
> should be queued after flush_delayed_work().

Yes, nothing should write after the device is closed but the events
are asynchronous so in theory the order is not guaranteed. I also find
it unlikely but I have no other theory at this point.

>
> > But
> > cancel_delayed_work_sync() at least makes sure the queue is empty so
> > the problem becomes less apparent.
> >
> > Could this explain what we're seeing?
>
> I suspect that cancel_delayed_work_sync() is only treating the symptoms
> by preventing the deferred work from running. The real bug is "someone"
> giving fb_deferred_io_work() invalid pages to work with. But that's
> just a blind guess.

Trying to figure out when the page goes away in relation to when the
work is triggered might be a good place to start.

>
> Best regards,
> Nam