Messages in this thread | | | Date | Tue, 29 Jan 2013 11:35:25 +0100 | From | Alexander Holler <> | Subject | Re: [PATCH 2/3 v2] fb: udlfb: fix hang at disconnect |
| |
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.
Regards,
Alexander
| |