On Wed, 18 Nov 2009, Jiri Kosina wrote: > On Tue, 17 Nov 2009, Russ Dill wrote: > > > Many usb drivers that create character devices use "struct > > usb_class_driver", a set of fops, and a usb_find_interface in their > > open call. A prime example is drivers/usb/usb-skeleton.c. A race > > occurs when userspace receives a hotplug event for the addition for > > the interface and then opens the associated device file before the > > device is added to the driver's klist_devices. > > > > The usb core senses a new usb device (usb_new_device) and calls > > device_add. This eventually gets down to really_probe and the > > usb-skeleton probe function, skel_probe. skel_probe calls > > usb_register_dev() which registers the associated character device for > > skel_class. The hotplug events for the class device get emitted. > > > > User space receives the hotplug event for the class device, makes the > > device node and notifies another program that opens the device node. > > The program opens the device node which calls into usb_open and then > > skel_open. skel_open calls usb_find_interface. usb_find_interfaces > > searches the klist_devices of skel_driver, finds no device associated > > with the minor number and returns NULL. skel_open returns -ENODEV. > > > > Control returns to really_probe and really_probe calls driver_bound > > which adds the device to the list of devices associated with > > skel_driver (klist_devices). > > > > I'm not sure what the right way to solve this is. A call to > > wait_for_device_probe() in the skel_open call before calling > > usb_find_interface fixes the problem, but it is a rather large hammer. I think the proper answer is for usb_find_interface() to use bus_for_each_dev() instead of driver_for_each_device(). Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
