linux-kernel - Re: [PATCH 2/3 v2] fb: udlfb: fix hang at disconnect

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Date:	Tue, 29 Jan 2013 12:11:11 +0100
From:	Alexander Holler <holler@...oftware.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux-kernel@...r.kernel.org, linux-fbdev@...r.kernel.org,
	Florian Tobias Schandinat <FlorianSchandinat@....de>,
	Bernie Thompson <bernie@...gable.com>,
	Steve Glendinning <steve.glendinning@...well.net>,
	stable@...r.kernel.org
Subject: Re: [PATCH 2/3 v2] fb: udlfb: fix hang at disconnect

Am 29.01.2013 11:35, schrieb Alexander Holler:
> Am 29.01.2013 01:56, schrieb Alexander Holler:
>> Am 29.01.2013 01:22, schrieb Andrew Morton:
>>> On Fri, 25 Jan 2013 19:49:27 +0100
>>> Alexander Holler <holler@...oftware.de> wrote:
>>>
>>>> When a device was disconnected the driver may hang at waiting for
>>>> urbs it never
>>>> will get. Fix this by using a timeout while waiting for the used
>>>> semaphore.
>>>>
>>>> There is still a memory leak if a timeout happens, but at least the
>>>> driver
>>>> now continues his disconnect routine.
>>>>
>>>> ...
>>>>
>>>> --- a/drivers/video/udlfb.c
>>>> +++ b/drivers/video/udlfb.c
>>>> @@ -1832,8 +1832,9 @@ static void dlfb_free_urb_list(struct
>>>> dlfb_data *dev)
>>>>       /* keep waiting and freeing, until we've got 'em all */
>>>>       while (count--) {
>>>>
>>>> -        /* Getting interrupted means a leak, but ok at disconnect */
>>>> -        ret = down_interruptible(&dev->urbs.limit_sem);
>>>> +        /* Timeout likely occurs at disconnect (resulting in a
>>>> leak) */
>>>> +        ret = down_timeout_killable(&dev->urbs.limit_sem,
>>>> +                        FREE_URB_TIMEOUT);
>>>>           if (ret)
>>>>               break;
>>>
>>> This is rather a hack.  Do you have an understanding of the underlying
>>> bug?  Why is the driver waiting for things which will never happen?
>
> To add a bit more explanation:
>
> I've experienced that bug after moving the fb-damage-handling into a
> workqueue (to make the driver usable as console). This likely has
> increased the possibility that an urb gets missed when the usb-stack
> calls the (usb-)disconnect function of the driver. But I don't know as I
> couldn't use the driver before (as fbcon) so I don't really have a
> comparison.
>
> What currently happens here is something like that:
>
> fb -> damage -> workload which sends urb and waits for answer
> device disconnect -> dlfb_usb_disconnect() -> stall (no answer to the
> above urb)
>
> I don't know why the disconnect waits for all urbs. The code looks like
> it does that just to free the allocated memory. As I'm not very familiar
> with the usb-stack, I would have to read up about the urb-handling to
> find out how to free the memory otherwise.
>
> As the previous comment in the code suggests that urbs already got
> missed (on shutdown) before, I assume that even without my patch, which
> moved the damage into a workqueue, the problem could occur which then
> prevents a shutdown as there is no timeout. As I've experienced that
> problem not only on disconnect, but on shutdown too (no shutdown was
> possible), I have to assume, that the previous used down_interruptible()
> didn't get a signal on shutdown (if the driver is used as fbcon),
> therefor I consider the timeout as necessary.

To explain the problem on shutdown a bit further, I think the following 
happens (usb and driver are statically linked and started by the kernel):

shutdown -> kill signal -> usb stack shuts down -> udlfb waits (forever) 
for a kill or an urb which it doesn't get.

Maybe the sequence is different if the usb-stack and udlfb are used as a 
module and/or udlfb is used only for X/fb. I'm not sure what actually 
does shut down the usb-stack in such a case, but maybe more than one 
kill signal might be thrown around.

Regards,

Alexander

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