Re: [PATCH] usb: typec: tcpm: fix tcpm unregister port but leave a pending timer

[Xu Yang <xu.yang_2@nxp.com>]

Hi,

> -----Original Message-----
> 
> On 11/18/21 5:18 AM, Heikki Krogerus wrote:
> > Hi,
> >
> > On Thu, Nov 18, 2021 at 05:23:52PM +0800, Xu Yang wrote:
> >> @@ -6428,6 +6432,9 @@ void tcpm_unregister_port(struct tcpm_port
> *port)
> >>   {
> >>      int i;
> >
> > You need to take the port lock here, no?
> >
> >          mutex_lock(&port->lock);
> >
> >> +    kthread_destroy_worker(port->wq);
> >> +    port->wq = NULL;
> >
> >          mutex_unlock(&port->lock);
> >
> 
> I don't think the timer code runs under the lock, so that won't help.
> But, yes, the code is inherently racy. At the very least, port->wq
> would have to be set to NULL first, but even that would have to be
> synchronized. I wonder how other drivers handle that situation.
> 
> Guenter
> 

Actually, the race is unlikely to happen if the hrtimer's expire time is a
little bigger. But it's better to avoid this situation from happening. So it
may be necessary to use a flag to indicate to the hrtimer the port is going
to be unregistered and it's no need to queue work anymore.

The patch maybe like following:

struct tcpm_port  *tcpm_register_port(...)
{
	...
	port->registered = true;
	return;
}

static enum hrtimer_restart  state_machine_timer_handler(timer)
{
	if (port->registered)
		kthread_queue_work(port->wq, work);
	return HRTIMER_NORESTART;
}

void tcpm_unregister_port(port)
{
	port->registered = false;
	kthread_destroy_worker(port->wq);

	hrtimer_cancel(&timer);
	...
}

What about this idea? Or I will submit it as patch V2.

Xu Yang