[PATCH] fix usb skeleton driver

[stefani]

From: Stefani Seibold <stefani@xxxxxxxxxxx>

This is a fix for the USB skeleton driver to bring it in shape.

- The usb_interface structure pointer will be no longer stored 
- Every access to the USB will be handled trought the usb_interface pointer
- Add a new bool 'connected' for signaling a disconnect (== false)
- Handle a non blocking read without blocking
- Code cleanup
- Synchronize disconnect() handler with open() and release(), to fix races
- Introduced fsync
- Single user mode
- Eliminated dead code
- Save some bytes in the dev structure

Some word about the open()/release() races with disconnect() of an USB device
(which can happen any time):
- The return interface pointer from usb_find_interface() could be already owned
  by an other driver and no more longer handle by the skeleton driver.
- Or the dev pointer return by usb_get_intfdata() could point to an already
  release memory.

This races can not handled by a per device mutex. Only a driver global mutex
could do this job, since the kref_put() in the skel_disconnect() must be
protected, otherwise skel_open() could access an already released memory.

I know that this races are very small, but on heavy load or misdesigned or buggy
hardware this could lead in a OOPS or unpredictable behavior.

The patch is against linux 3.5.0 commit eea5b5510fc5545d15b69da8e485a7424ae388cf

Hope this helps

Signed-off-by: Stefani Seibold <stefani@xxxxxxxxxxx>
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