Subj : Freeing data structures in a threaded application
To   : comp.programming.threads
From : Luke  Ehresman
Date : Wed Feb 16 2005 06:29 am

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)

I am using C with PThreads, if that is relevant.
Thanks,
Luke

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