linux-kernel - Race condition between userland and USB device attachment

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Date:	Fri, 25 Jul 2014 17:43:26 +0400
From:	Sergey Klyaus <Sergey.Klyaus@...e-IT.Ru>
To:	linux-kernel@...r.kernel.org
CC:	Sergey Klyaus <Sergey.Klyaus@...e-IT.ru>
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.
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