Subj : Re: A question about atomic_ptr
To   : comp.programming.threads
From : Joe Seigh
Date : Sat Apr 16 2005 10:12 am

On Sat, 16 Apr 2005 15:52:09 +0300, Peter Dimov <pdimov@gmail.com> wrote:
> For me, the most painful part of a hazard pointer implementation is the
> pointer-per-thread table (ac_thread.c). (There are some issues with
> destruction and deallocation but they probably can be solved.)

Making the table dynamically resizable would be a little tricky but
doable.  I don't think you'd need more than one or two hazard pointers
though.  Big problem is getting rid of the hazard pointer memory barrier
but that's doable also.

>
> But there's one other thing: shared_ptr is two pointers, not one. Without
> DWCAS, or at least double-wide atomic loads and stores, even hazards aren't
> enough to make it atomic. :-(
>

Sounds like it's using separate refcount and object pointers to make dereferencing
the object less expensive, as opposed to keeping the object pointer with the
refcount which is what I do with atomic_ptr.  It's an implementation issue.
If you want atomicity you do whatever it takes.  You need a compiler that's
smarter about optimization maybe.

-- 
Joe Seigh

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