Subj : Re: Freeing data structures in a threaded application
To   : comp.programming.threads
From : Joe Seigh
Date : Wed Feb 16 2005 04:26 pm

On 16 Feb 2005 06:29:45 -0800, Luke  Ehresman <lehresma@gmail.com> wrote:

> Hello,
>
> I have a program with several threads.  A session is created for a user
> in one thread and stays around for 600 seconds and will then time out
> with inactivity (assuming the user doesn't use the session in 600
> seconds).  At that time, one of my threads will free up the session.
>
> The potential problem I have noticed is that I have another thread that
> does stuff to the session in the background.  I was thinking this
> morning, what happens if the session is freed up while the cache
> manager thread is working on that same session?  I do have a mutex to
> lock the session, but that mutex is in the session data structure
> itself, so once the structure is freed, the mutex goes away.
>
> The only way I can figure to get around this is to have a mutex outside
> of the session data structure.  But then how do I go about freeing up
> that mutex after the session is invalid?  It seems I would run into the
> same problem.  I can't keep around global mutexes because there may
> potentially be thousands of users using this, logging in and out all
> the time.  (i.e. it needs to be scalable)
>
You can use reference counting to protect the session data.  You only
free the session data when the last reference is dropped.

-- 
Joe Seigh

.