[Bordeaux-threads-devel] Clozure: fix condition-wait

[James M. Lawrence]

On Tue, Jul 3, 2012 at 9:11 AM, Martin Simmons <martin at lispworks.com> wrote:
>>>>>> On Sat, 30 Jun 2012 15:10:09 -0400, James M Lawrence said:
>>
>> If release-lock fails then do not call acquire-lock.
>
> What can cause release-lock to fail?  Or do you mean that something might do
> an async throw on entry to release-lock?  If so, what prevents an async throw
> on exit from release-lock in the new version (causing the opposite bug)?

release-lock signals an error when the lock is not owned. Without the
patch, when condition-wait fails in this manner it silently acquires
the lock. To complicate matters, CCL lacks non-recursive locks
(consecutive acquire-lock calls do not cause an error), so this local
failure can cause a global problem which is not immediately detected.

If we are talking about interrupts then there are problems in either
case, both before and after the patch. Making condition-wait robust in
the face of interrupts is a different issue, and may not be possible
here. One solution might be the following, although it may have
implications of its own.

(defun condition-wait (condition-variable lock)
  (ccl:without-interrupts
    (release-lock lock)
    (unwind-protect
         (ccl:with-interrupts-enabled
           (ccl:wait-on-semaphore condition-variable))
      (acquire-lock lock))))