Messages in this thread | | | Date | Tue, 29 Jan 2013 12:11:11 +0100 | From | Alexander Holler <> | Subject | Re: [PATCH 2/3 v2] fb: udlfb: fix hang at disconnect |
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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@ahsoftware.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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