Man page 9 : usb_kill_urb

usb_kill_urb - cancel a transfer request and wait for it to finish

void usb_kill_urb(struct urb *  urb );

urb
    pointer to URB describing a previously submitted request, may be NULL

This routine cancels an in-progress request. It is guaranteed that upon return all completion handlers will have finished and the URB will be totally idle and available for reuse. These features make this an ideal way to stop I/O in a disconnect callback or close function. If the request has not already finished or been unlinked the completion handler will see urb->status == -ENOENT.

While the routine is running, attempts to resubmit the URB will fail with error -EPERM. Thus even if the URBAqs completion handler always tries to resubmit, it will not succeed and the URB will become idle.

The URB must not be deallocated while this routine is running. In particular, when a driver calls this routine, it must insure that the completion handler cannot deallocate the URB.

This routine may not be used in an interrupt context (such as a bottom half or a completion handler), or when holding a spinlock, or in other situations where the caller canAqt schedule.

This routine should not be called by a driver after its disconnect method has returned.