Messages in this thread | | | Date | Fri, 25 Jul 2014 17:43:26 +0400 | From | Sergey Klyaus <> | Subject | Race condition between userland and USB device attachment |
| |
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).
P.S. Since I aren't subscribed to mailing list, could you add me in CC?
Thanks in advance. Best Regards, Sergey.
| |