Re: [9fans] stack reclamation on Unix using clone, rfork_thread, etc.

[Philippe Anel <xigh@free.fr>]

>Yes, but what thread does the free?  You can't free your own stack, and
>how does any other thread know to do it for you?  This is a pain, and a
>pain that I have felt myself.
>
>Of course, it's easy if there is some kind of termination
>synchronization, and the thread that waits is responsible for cleaning
>up after the thread that finishes.  But I assume that Russ is trying to
>solve the problem in the general case.
>
>As far as I know, the new pthreads library on Linux uses a technique
>very similar to the one that Russ describes, with the additional
>optimization of re-using stacks if possible.  In fact, I'm not sure if
>they ever free stacks -- just keep them around in case a new thread
>needs one.
>
>The old linuxthreads library used a "manager thread" to do the reaping
>(among other things), which isn't particularly nice or efficient.

Ok. What do you think then about the following pseudo-code:

------------------------------------------------------------------------------
struct segment {
         struct segment * next;
         struct segment * prev;
         void * addr;
         int len;
         int threadid;
};

struct addr_space {
         Lock;
         int refcount;
         struct segment * inuse;
         struct segment * tofree;
} addrspace;

void
exit_thread()
  {
         struct segment * s;

         lock(&addrspace);
         for (s = addrspace.tofree; s; s = s->next)
                 free(s->addr);
         if (--refcount) {
                 s = find_segment_from_id(get_my_thread_id());
                 list_remove(s);
                 list_add(tofree, s);
                 unlock(&addrspace);
         }
}

------------------------------------------------------------------------------

It would free the stack of the previous exited thread.

         Philippe,