On 12/09/2014 04:24 PM, 'Greg Kroah-Hartman' wrote: > On Mon, Dec 08, 2014 at 09:44:05AM +0000, David Laight wrote: >> From: Greg Kroah-Hartman >>> On Fri, Dec 05, 2014 at 09:03:57PM +0100, Sebastian Andrzej Siewior wrote: >>>> Consider the following scenario: >>>> - plugin a webcam >>>> - play the stream via gst-launch-0.10 v4l2src device=/dev/video0 >>>> - remove the USB-HCD during playback via "rmmod $HCD" >>>> >>>> and now wait for the crash >>> >>> Which you deserve, why did you ever remove a kernel module? That's racy >>> and _never_ recommended, which is why it never happens automatically and >>> only root can do it. >> >> Really drivers and subsystems should have the required locking (etc) to >> ensure that kernel modules can either be unloaded, or that the unload >> request itself fails if the device is busy. >> >> It shouldn't be considered a 'shoot self in foot' operation. >> OTOH there are likely to be bugs. > > This is not always the case, sorry, removing a kernel module is a known > racy condition, and sometimes adding all of the locking required to try > to make it "safe" just isn't worth it overall, as this is something that > _only_ a developer does. I wasn't are of that. rmmod does not mention this. Kconfig does not mention this and suggest y as default (for MODULE_UNLOAD) . rmmod -f likely causes problems but this is not the case here. If you want to avoid rmmod why not mark a driver that it is not safe to remove it? And why not make it work? You can unbind the HCD driver from the PCI-device via sysfs and this is not something not only a developer does. This "unbind" calls the remove function of the driver and the only difference between unbind and rmmod is that the module remains inserted (but this is no news for you). Now, this unbind happens if you choose to pass a PCI-device to a qemu guest. This is a fairly common use-case for a non-developer since it is quite easy to setup in virt-manager for instance. All you need is a hardware with IOMMU support. I used this to get the usb.org testsuite running in my Windows guest which needs access to EHCI registers). I could also mention hacking on XHCI and not crashing the physical machine if something goes south but then I would rise the developer card again. I even rmmod & modprobe my mmc controller on my notebook because for some reason it does not work otherwise after a suspend + resume cycle (and my motivation to look after this is quite low since I barely use my notebook at all). I am really surprised that you as a core developer and maintainer of the drivers infrastructure say that one should not remove a driver. > greg k-h Sebastian -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
