anchor.diff:  
   (defun irq-fifo-reset (fifo)
     "Flush any waiting data."
  +  (check-type fifo irq-fifo)
     (safe-without-interrupts (fifo)
       (with-place-spinlock ((irq-fifo-lock fifo))
         (setf (irq-fifo-head fifo) 0
  
positive.code: Returns two values. The first value is the value popped from the FIFO.
  The second value is true if a value was popped, false otherwise.
  It is only possible for the second value to be false when wait-p is false."
    (loop
         (multiple-value-bind (value validp)
             (safe-without-interrupts (fifo)
  
negative.code:   (contested-count 0))
  
  (defun acquire-mutex (mutex &optional (wait-p t))
    (check-type mutex mutex)
    (let ((self (current-thread)))
      ;; Fast path - try to lock.
      (when (eql (sys.int::cas (mutex-state mutex) :unlocked :locked) :unlocked)
  