[threads-standard-discuss] MP standardization proposal

[Gilbert Baumann]

Robert Strandh writes:
 > I am having a hard time understanding condition variables. 
 > 
 > > By providing an atomic "release lock and sleep" operation, a condition
 > > variable guards against the "lost notification" problem, which is
 > > where the writer adds data to the buffer in between the time that a
 > > reader tests it (finding it empty) and goes to sleep.
 > 
 > I do not see how having a "release lock and sleep" operation fixes the
 > "lost notification" problem.  Who would use the operation?  The reader
 > or the writer or both?  
 > 

The common use case is like this:

(defun reader ()
  (with-lock-held (*lock*)
    (loop
      (condition-wait *condvar* *lock*)
      (when <something there>
         <consume>))))

(defun writer ()
  (loop
    (with-lock-held (*lock*)
      <add something to be consumed>
      (condition-notify *condvar*))))

The notification cannot be lost since entering CONDITION-WAIT is
while the lock is held at which point the writer cannot have it. Only
after things are set up within the deep magic of CONDITION-WAIT to
actually wait for the condition to be notified the lock is released.

Or put the other way round: The reader only releases the lock while
being blocked within CONDITION-WAIT. The writer can only notify while
holding the lock. 

=> CONDITION-NOTIFY can only be called when the reader is blocked in
   CONDITION-WAIT
=> no notifications get lost.

qed :)

-- 
Gilbert Baumann                            Base Electronic GmbH, Norderstedt
gilbert at base-electronic.de                    http://www.base-electronic.de/