linux-kernel - Re: [PATCH 1/4] usb: hub: convert khubd into workqueue

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Message-ID: <Pine.LNX.4.44L0.1409121011470.1180-100000@iolanthe.rowland.org>
Date:	Fri, 12 Sep 2014 10:16:21 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Petr Mladek <pmladek@...e.cz>
cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Tejun Heo <tj@...nel.org>,
	Joe Lawrence <joe.lawrence@...atus.com>,
	Jiri Kosina <jkosina@...e.cz>, <linux-usb@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] usb: hub: convert khubd into workqueue

On Fri, 12 Sep 2014, Petr Mladek wrote:

> There is no need to have separate kthread for handling USB hub events.
> It is more elegant to use the workqueue framework.
> 
> The workqueue is allocated as unbound, cpu intensive, and freezable.
> There does not seem to be any big advantage to run it on the same CPU.
> The handler is taking a lock and thus could block for a longer time.
> And finally, the original thread was freezable as well.
> 
> struct usb_hub is passed via the work item. Therefore we do not need
> hub_event_list.
> 
> hub_events() is modified to process the given work item. It is renamed to
> hub_event(). The while cycle will be removed in a followup patch. It helps
> to see the real change here.
> 
> One nice thing is that we do not need hub_event_lock any longer. It was needed
> when doing operations with hub_event_list and for balancing the calls
> usb_autopm_get_interface_no_resume() and usb_autopm_put_interface_no_suspend().
> It still works because the workqueue operations have their own locking.
> Also cancel_work_sync() tells us whether any work item was canceled.
> It means that we could put the interface either in hub_event() handler or when
> the work item was successfully canceled.

I don't think you can eliminate the lock quite so easily.  This patch 
introduces some nasty races.

> @@ -577,18 +571,20 @@ static int hub_port_status(struct usb_hub *hub, int port1,
>  
>  static void kick_khubd(struct usb_hub *hub)
>  {
> -	unsigned long	flags;
> -
> -	spin_lock_irqsave(&hub_event_lock, flags);
> -	if (!hub->disconnected && list_empty(&hub->event_list)) {
> -		list_add_tail(&hub->event_list, &hub_event_list);
> -
> -		/* Suppress autosuspend until khubd runs */
> +	if (!hub->disconnected && !work_pending(&hub->events)) {

Here you test hub->disconnected, with no lock for protection.

(Also, note that work_pending is not synchronized with anything.  What 
happens if two threads call this routine at the same time?)

> @@ -1647,13 +1643,9 @@ static void hub_disconnect(struct usb_interface *intf)
>  	int port1;
>  
>  	/* Take the hub off the event list and don't let it be added again */
> -	spin_lock_irq(&hub_event_lock);
> -	if (!list_empty(&hub->event_list)) {
> -		list_del_init(&hub->event_list);
> +	if (cancel_work_sync(&hub->events))
>  		usb_autopm_put_interface_no_suspend(intf);
> -	}
>  	hub->disconnected = 1;

And here you set hub->disconnected with no lock for protection.  So 
what happens if one thread calls kick_khubd at the same time as another 
thread calls hub_disconnect?

Alan Stern

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