Re: [PATCH v2] usb: ucsi: fix connector partner ucsi work issue

[Heikki Krogerus <heikki.krogerus@linux.intel.com>]

Hi,

On Thu, Jan 05, 2023 at 10:34:41AM -0800, Jack Pham wrote:
> On Thu, Jan 05, 2023 at 01:27:07PM +0200, Heikki Krogerus wrote:
> > On Thu, Jan 05, 2023 at 01:42:40PM +0800, Linyu Yuan wrote:
> > > When a PPM client unregisters with UCSI framework, connector specific work
> > > queue is destroyed. However, a pending delayed work queued before may
> > > still exist. Once the delay timer expires and the work is scheduled,
> > > this can cause a system crash as the workqueue is destroyed already.
> > 
> > When the workqueue is destroyed it's also flushed, no? So how could
> > there be still something pending, delayed or not? Did you actually see
> > this happening?
> 
> Yes, we encountered this during a scenario in which our PPM firmware 
> is undergoing a reset which is handled by calling ucsi_unregister().
> The connectors' workqueues are destroyed but apparently the
> destroy_workqueue() does *not* seem to take care pending delayed items.
> 
> 	Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
> 	...
> 	Call trace:
> 	 __queue_work+0x1f4/0x550
> 	 delayed_work_timer_fn+0x28/0x38
> 	 call_timer_fn+0x3c/0x238
> 	 expire_timers+0xcc/0x168
> 	 __run_timers+0x194/0x1f8
> 	 run_timer_softirq+0x2c/0x54
> 	 _stext+0xec/0x3a8
> 	 __irq_exit_rcu+0xa0/0xfc
> 	 irq_exit_rcu+0x18/0x28
> 	 el0_interrupt+0x5c/0x138
> 	 __el0_irq_handler_common+0x20/0x30
> 	 el0t_64_irq_handler+0x18/0x28
> 	 el0t_64_irq+0x1a0/0x1a4
> 	Code: eb16013f 54000300 aa1a03e0 9441be2a (f9400280) 
> 
> In particular this is happening for the ucsi_check_connection() which is
> the currently the only work item using a non-zero delay.  If we look
> closely at queue_delayed_work() we can see in that case it defers by
> using a separate timer:
> 
> 	static void __queue_delayed_work(int cpu, struct workqueue_struct *wq,
> 					struct delayed_work *dwork, unsigned long delay)
> 	{
> 		struct timer_list *timer = &dwork->timer;
> 		struct work_struct *work = &dwork->work;
> 
> 		<...snip...>
> 
> 		/*
> 		 * If @delay is 0, queue @dwork->work immediately.  This is for
> 		 * both optimization and correctness.  The earliest @timer can
> 		 * expire is on the closest next tick and delayed_work users depend
> 		 * on that there's no such delay when @delay is 0.
> 		 */
> 		if (!delay) {
> 			__queue_work(cpu, wq, &dwork->work);
> 			return;
> 		}
> 
> 		dwork->wq = wq;
> 		       ^^^^^^^^ wq gets saved here, but might be
> 				destroyed before timer expires
> 
> 		dwork->cpu = cpu;
> 		timer->expires = jiffies + delay;
> 
> 		if (unlikely(cpu != WORK_CPU_UNBOUND))
> 			add_timer_on(timer, cpu);
> 		else
> 			add_timer(timer);
> 	}
> 
> In ucsi_unregister() we destroy the connector's wq, but there is a
> pending timer still for the ucsi_check_connection() item and upon
> expiry it tries to do the real __queue_work() call on a dangling 'wq'.

Okay.

> > > Fix this by moving all partner related delayed work to connector instance
> > > and cancel all of them when ucsi_unregister() is called by PPM client.
> > 
> > I would love to be able to cancel these works, though not because of
> > driver removal - I'm more concerned about disconnections. The reason
> > why each task is a unique work is because it allows the driver to add
> > the same task to the queue as many times as needed, and that was
> > needed inorder to recover from some firmware issues. If there's only a
> > dedicated work per task like in your proposal, we can only schedule
> > the task once at a time, and that may lead into other issues.
> 
> I see, queuing a task multiple times is a good reason to allocate the
> workers dynamically.  Then what we really need is a way to reliably
> cancel & reclaim any pending items that are sitting on their own timers,
> since they are otherwise unreachable via the 'wq'. 
> 
> cancel_delayed_work(), cancel_delayed_work_sync(), flush_delayed_work()
> all require the handle to the delayed_work itself which we don't keep a
> reference to.
> 
> Do you have any other suggestions for this?

How about separate list for the works?

diff --git a/drivers/usb/typec/ucsi/ucsi.c b/drivers/usb/typec/ucsi/ucsi.c
index 8695eb2c6227e..d5cf7573a2cfa 100644
--- a/drivers/usb/typec/ucsi/ucsi.c
+++ b/drivers/usb/typec/ucsi/ucsi.c
@@ -187,6 +187,7 @@ EXPORT_SYMBOL_GPL(ucsi_send_command);
 
 struct ucsi_work {
        struct delayed_work work;
+       struct list_head node;
        unsigned long delay;
        unsigned int count;
        struct ucsi_connector *con;
@@ -202,6 +203,7 @@ static void ucsi_poll_worker(struct work_struct *work)
        mutex_lock(&con->lock);
 
        if (!con->partner) {
+               list_del(&uwork->node);
                mutex_unlock(&con->lock);
                kfree(uwork);
                return;
@@ -209,10 +211,12 @@ static void ucsi_poll_worker(struct work_struct *work)
 
        ret = uwork->cb(con);
 
-       if (uwork->count-- && (ret == -EBUSY || ret == -ETIMEDOUT))
+       if (uwork->count-- && (ret == -EBUSY || ret == -ETIMEDOUT)) {
                queue_delayed_work(con->wq, &uwork->work, uwork->delay);
-       else
+       } else {
+               list_del(&uwork->node);
                kfree(uwork);
+       }
 
        mutex_unlock(&con->lock);
 }
@@ -236,6 +240,7 @@ static int ucsi_partner_task(struct ucsi_connector *con,
        uwork->con = con;
        uwork->cb = cb;
 
+       list_add_tail(&uwork->node, &con->works);
        queue_delayed_work(con->wq, &uwork->work, delay);
 
        return 0;
@@ -1056,6 +1061,7 @@ static int ucsi_register_port(struct ucsi *ucsi, int index)
        INIT_WORK(&con->work, ucsi_handle_connector_change);
        init_completion(&con->complete);
        mutex_init(&con->lock);
+       INIT_LIST_HEAD(&con->works);
        con->num = index + 1;
        con->ucsi = ucsi;
 
diff --git a/drivers/usb/typec/ucsi/ucsi.h b/drivers/usb/typec/ucsi/ucsi.h
index 8361e1cfc8eba..dcb792eeedb94 100644
--- a/drivers/usb/typec/ucsi/ucsi.h
+++ b/drivers/usb/typec/ucsi/ucsi.h
@@ -325,6 +325,7 @@ struct ucsi_connector {
        struct work_struct work;
        struct completion complete;
        struct workqueue_struct *wq;
+       struct list_head works;
 
        struct typec_port *port;
        struct typec_partner *partner;


Something like that. Then just walk through the list and cancel each
work in it before destroying the wq. Would that work?

thanks,

-- 
heikki