Messages in this thread | | | Date | Mon, 28 Jul 2014 10:22:52 -0400 (EDT) | From | Alan Stern <> | Subject | Re: Race condition between userland and USB device attachment |
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On Mon, 28 Jul 2014, Oliver Neukum wrote:
> On Fri, 2014-07-25 at 17:43 +0400, Sergey Klyaus wrote: > > Hello. > > > > I am currently working on a project with Thin clients with Citrix > > Receiver 13 for Linux and encountered interesting problem with USB > > device redirection. > > ctxusb/ctxusbd process from Citrix Receiver are using inotify mechanism > > to monitor /dev/bus/usb filesystem, and when device arrives, tries to > > open it, but get ENODEV status: > > Jul 25 11:36:13 myaut-desktop ctxusbd[2664]: Failed to open device: No > > such device > > Jul 25 11:36:13 myaut-desktop ctxusb[2751]: Failed to open device > > 001:003 (error 19 - No such device), bad id? > > > > It is caused by design of device_add() function: it calls > > devtmpfs_create_node before bus_add_device. Here are sequence of events: > > 1. device_add() calls devtmpfs_create_node(). That leads to inotify > > event that. > > 2. ctxusb is awoken because inotify event arises, and calls ctxusbd daemon. > > 3. ctxusbd daemon opens /dev/bus/usb/new-device, so usbdev_open() > > routine is called > > 4. usbdev_open() calls usbdev_lookup_by_devt(). Because device is not > > yet attached to "usb bus", it returns NULL, and thus usbdev_open() > > returns -ENODEV > > 5. Finally, device_add() calls bus_add_device(), and all subsequent > > calls of usbdev_open() will succeed. However, ctxusb/ctxusbd already > > reported an error and abandon device. User is unsatisfied. > > > > I was able to reproduce that issue on Ubuntu 10.04 with 2.6.32 and 3.13 > > kernels. > > However, it only occur on uni-processor systems (!) > > > > I see three ways to solve that issue: > > 1. Leave it to userland applications (i.e. using loop with retries and > > timeouts). However, I feel that it is a kernel issue (application is > > notified before device is ready). > > 2. Call bus_add_device() before devtmpfs_create_node(). Very rough, and > > probably breaks a lot of other kernel code. > > 3. Wait in usbdev_open() until reconfiguration is finished (i.e. by > > using some global lock between usb_new_device() and usbdev_open(), or > > add completion and special state USB_STATE_CONNECTING to a device). > > No to your third option. This is no USB problem. The issue is in the > generic code. The only clean fix is your suggestion (2)
I agree. That whole "if (MAJOR(dev->devt)) {" thing in device_add() should come at the end, not in the middle.
Alan Stern
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