Merge tag 'usb-3.4-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb

Pull USB fixes from Greg KH:
 "Here are a number of fixes for the USB core and drivers for 3.4-rc2

  Lots of tiny xhci fixes here, a few usb-serial driver fixes and new
  device ids, and a smattering of other minor fixes in different USB
  drivers.

  Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>"

* tag 'usb-3.4-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb: (30 commits)
  USB: update usbtmc api documentation
  xHCI: Correct the #define XHCI_LEGACY_DISABLE_SMI
  xHCI: use gfp flags from caller instead of GFP_ATOMIC
  xHCI: add XHCI_RESET_ON_RESUME quirk for VIA xHCI host
  USB: fix bug of device descriptor got from superspeed device
  xhci: Fix register save/restore order.
  xhci: Restore event ring dequeue pointer on resume.
  xhci: Don't write zeroed pointers to xHC registers.
  xhci: Warn when hosts don't halt.
  xhci: don't re-enable IE constantly
  usb: xhci: fix section mismatch in linux-next
  xHCI: correct to print the true HSEE of USBCMD
  USB: serial: fix race between probe and open
  UHCI: hub_status_data should indicate if ports are resuming
  EHCI: keep track of ports being resumed and indicate in hub_status_data
  USB: fix race between root-hub suspend and remote wakeup
  USB: sierra: add support for Sierra Wireless MC7710
  USB: ftdi_sio: fix race condition in TIOCMIWAIT, and abort of TIOCMIWAIT when the device is removed
  USB: ftdi_sio: fix status line change handling for TIOCMIWAIT and TIOCGICOUNT
  USB: don't ignore suspend errors for root hubs
  ...

1 files changed, 22 insertions, 0 deletions

diff --git a/Documentation/usb/URB.txt b/Documentation/usb/URB.txt
index 8ffce746d496..00d2c644068e 100644
--- a/Documentation/usb/URB.txt
+++ b/Documentation/usb/URB.txt
@@ -168,6 +168,28 @@ that if the completion handler or anyone else tries to resubmit it
 they will get a -EPERM error.  Thus you can be sure that when
 usb_kill_urb() returns, the URB is totally idle.
 
+There is a lifetime issue to consider.  An URB may complete at any
+time, and the completion handler may free the URB.  If this happens
+while usb_unlink_urb or usb_kill_urb is running, it will cause a
+memory-access violation.  The driver is responsible for avoiding this,
+which often means some sort of lock will be needed to prevent the URB
+from being deallocated while it is still in use.
+
+On the other hand, since usb_unlink_urb may end up calling the
+completion handler, the handler must not take any lock that is held
+when usb_unlink_urb is invoked.  The general solution to this problem
+is to increment the URB's reference count while holding the lock, then
+drop the lock and call usb_unlink_urb or usb_kill_urb, and then
+decrement the URB's reference count.  You increment the reference
+count by calling
+
+	struct urb *usb_get_urb(struct urb *urb)
+
+(ignore the return value; it is the same as the argument) and
+decrement the reference count by calling usb_free_urb.  Of course,
+none of this is necessary if there's no danger of the URB being freed
+by the completion handler.
+
 
 1.7. What about the completion handler?