ohci-hcd: Fix race condition caused by ohci_urb_enqueue() and io_watchdog_func()

Running io_watchdog_func() while ohci_urb_enqueue() is running can
cause a race condition where ohci->prev_frame_no is corrupted and the
watchdog can mis-detect following error:

  ohci-platform 664a0800.usb: frame counter not updating; disabled
  ohci-platform 664a0800.usb: HC died; cleaning up

Specifically, following scenario causes a race condition:

  1. ohci_urb_enqueue() calls spin_lock_irqsave(&ohci->lock, flags)
     and enters the critical section
  2. ohci_urb_enqueue() calls timer_pending(&ohci->io_watchdog) and it
     returns false
  3. ohci_urb_enqueue() sets ohci->prev_frame_no to a frame number
     read by ohci_frame_no(ohci)
  4. ohci_urb_enqueue() schedules io_watchdog_func() with mod_timer()
  5. ohci_urb_enqueue() calls spin_unlock_irqrestore(&ohci->lock,
     flags) and exits the critical section
  6. Later, ohci_urb_enqueue() is called
  7. ohci_urb_enqueue() calls spin_lock_irqsave(&ohci->lock, flags)
     and enters the critical section
  8. The timer scheduled on step 4 expires and io_watchdog_func() runs
  9. io_watchdog_func() calls spin_lock_irqsave(&ohci->lock, flags)
     and waits on it because ohci_urb_enqueue() is already in the
     critical section on step 7
 10. ohci_urb_enqueue() calls timer_pending(&ohci->io_watchdog) and it
     returns false
 11. ohci_urb_enqueue() sets ohci->prev_frame_no to new frame number
     read by ohci_frame_no(ohci) because the frame number proceeded
     between step 3 and 6
 12. ohci_urb_enqueue() schedules io_watchdog_func() with mod_timer()
 13. ohci_urb_enqueue() calls spin_unlock_irqrestore(&ohci->lock,
     flags) and exits the critical section, then wake up
     io_watchdog_func() which is waiting on step 9
 14. io_watchdog_func() enters the critical section
 15. io_watchdog_func() calls ohci_frame_no(ohci) and set frame_no
     variable to the frame number
 16. io_watchdog_func() compares frame_no and ohci->prev_frame_no

On step 16, because this calling of io_watchdog_func() is scheduled on
step 4, the frame number set in ohci->prev_frame_no is expected to the
number set on step 3.  However, ohci->prev_frame_no is overwritten on
step 11.  Because step 16 is executed soon after step 11, the frame
number might not proceed, so ohci->prev_frame_no must equals to
frame_no.

To address above scenario, this patch introduces a special sentinel
value IO_WATCHDOG_OFF and set this value to ohci->prev_frame_no when
the watchdog is not pending or running.  When ohci_urb_enqueue()
schedules the watchdog (step 4 and 12 above), it compares
ohci->prev_frame_no to IO_WATCHDOG_OFF so that ohci->prev_frame_no is
not overwritten while io_watchdog_func() is running.

Signed-off-by: Shigeru Yoshida <Shigeru.Yoshida@windriver.com>
Signed-off-by: Haiqing Bai <Haiqing.Bai@windriver.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>