SBCL exports a symbol sb-ext:compare-and-swap, which can be used to do small-scale transactional tricks, and is often an order of magnitude faster than the equivalent locking solution. Here is a very simple concurrency-safe stack. The macro is a convenient wrapper that helps use compare-and-swap in a 'transactional' way. http://marijnhaverbeke.n…

make-cstack.lisp

(defmacro concurrent-update (place var &body body)
  `(flet ((action (,var) ,@body))
     (let ((prev ,place))
       (loop :until (eq (sb-ext:compare-and-swap ,place prev (action prev)) prev)
             :do (setf prev ,place))
       prev)))

(defun make-cstack (&rest elements)
  (cons :stack elements))
(defun push-cstack (element stack)
  (concurrent-update (cdr stack) elts
    (cons element elts))
  (values))
(defun pop-cstack (stack)
  (car (concurrent-update (cdr stack) elts
         (cdr elts))))