On Thu, May 28, 2020 at 09:51:35PM +0200, Andrey Konovalov wrote: > On Thu, May 28, 2020 at 9:40 PM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > > > > On Thu, May 28, 2020 at 09:03:43PM +0200, Andrey Konovalov wrote: > > > > > Ah, so the problem is that when a process exits, it tries to close wdm > > > fd first, which ends up calling wdm_flush(), which can't finish > > > because the USB requests are not terminated before raw-gadget fd is > > > closed, which is supposed to happen after wdm fd is closed. Is this > > > correct? I wonder what will happen if a real device stays connected > > > and ignores wdm requests. > > > > > > I don't understand though, how using wait_event_interruptible() will > > > shadow anything here. > > > > > > Alan, Greg, is this acceptable behavior for a USB driver? > > > > I don't understand what the problem is. Can you explain in more general > > terms -- nothing specific to wdm or anything like that -- what you are > > concerned about? Is this something that could happen to any gadget > > driver? Or any USB class device driver? Or does it only affect > > usespace components of raw-gadget drivers? > > So, AFAIU, we have a driver whose flush() callback blocks on > wait_event(), which can only terminate when either 1) the driver > receives a particular USB response from the device or 2) the device > disconnects. This sounds like a bug in the driver. What would it do if someone had a genuine (not emulated) but buggy USB device which didn't send the desired response? The only way to unblock the driver would be to unplug the device! That isn't acceptable behavior. > For 1) the emulated device doesn't provide required > responses. For 2) the problem is that the emulated via raw-gadget > device disconnects when the process is killed (and raw-gadget fd is > closed). But that process is the same process that is currently stuck > on wait_event() in the flush callback(), and therefore unkillable. What would happen if you unload dummy-hcd at this point? Or even just do: echo 0 >/sys/bus/usb/devices/usbN/bConfigurationValue, where N is the bus number of the dummy-hcd bus? > This can generally happen with any driver that goes into > uninterruptible sleep within one of its code paths reachable from > userspace that can only be unblocked by a particular behavior from the > USB device. But I haven't seen any such drivers so far, wdm is the > first. Drivers should never go into uninterruptible sleep states unless they can guarantee that the duration will be bounded somehow (for example, by a reasonable timeout). Or that cutting the sleep state short would cause the system to crash -- but that's not an issue here. Alan Stern
