Subj : Re: Freeing data structures in a threaded application
To   : comp.programming.threads
From : David Schwartz
Date : Wed Feb 16 2005 06:56 am


"Luke Ehresman" <lehresma@gmail.com> wrote in message 
news:1108564948.066645.184270@o13g2000cwo.googlegroups.com...

> Thanks for the tips.  I do have a mutex for the session list for adding
> and removing sessions from that list.  However, I also have separate
> mutexes for operating on individual sessions.  That way I can do
> operations on multiple sessions at the same time (but can only modify
> the session list one at a time).

    That's good.

> Your idea of keeping a list of sessions that are pending deletion is a
> great idea.  That's the piece I was missing.

    Are you sure you need one? Why not just acquire the lock that protects 
the list of sessions and delete the session?

    Just make sure any code that needs the lock on the list of sessions 
releases any locks on individual sessions first. So the code looks like 
this:

mySession->Lock(); // lock this session
if(mySession->NeedToDelete())
{ // we need to delete it
 mySession->Unlock();
 SessionTable->Lock(); // trade for the session table lock, our session 
might not exist anymore
 if(SessionTable()->FindSession(mySession))
 { // our session still exists
  SessionTable()->Remove(mySession); // remove this session
  SessionTable->Unlock();
  // since we removed the session from the table, nobody else did
  // therefore, nobody else could have deleted it, so it must still exist
  mySession()->Lock();
  // at this point, we hold the lock on the session and it is not in the 
session list
  // no other thread could possibly reference this session
  if(mySession()->NeedToDelete())
  {
   mySession()->Unlock();
   delete mySession();
  }
  else
  {
    // wow, there must have been some activity
    mySession()->Unlock();
  }
 }
 else
 {
  // wow, the cache manager must have deleted it right before we could
  SessionTable()->Unlock();
 }
}
else mySession.Unlock();

    Not pretty, but variations on this may work very well for your 
particular situation.

    DS

.