Re: [boost] storing intrusive_ptr in atomic?!

Subject: Re: [boost] storing intrusive_ptr in atomic?!
From: Giovanni Piero Deretta (gpderetta_at_[hidden])
Date: 2013-10-25 08:12:07

On Fri, Oct 25, 2013 at 12:43 PM, Oliver Kowalke
<oliver.kowalke_at_[hidden]>wrote:

> 2013/10/25 Giovanni Piero Deretta <gpderetta_at_[hidden]>
>
> > that's the tricky part. To adjust the reference count, you must load and
> > dereference the pointer itself, but between those two operations another
> > thread might come in, replace the pointer, adjust the original pointer
> > count down and free the pointed object, together with the counter. You
> need
> > some way to defer destruction till a safe point (RCU, hazard pointers,
> > etc).
> >
>
> what about this
>
> // use-counter in T is an atomic too
> array< atomic< intrusive_ptr< T > >, 100 > a;
>
> intrusive_ptr< T > p(...);
> intrusive_ptr_add_ref( p.get() );
> // add an intrusive_ptr to the array
> if ( a[index].compare_exchange_strong(null_ptr,p) ) {
>   // successful added
> }
>
> // remove an interusive_ptr from array
> intrusive_ptr< T > e; // points to nothing
> if ( ! a[index].compare_exchange_strong(e, null_ptr) ) {
>   // successfull removed
>   intrusive_ptr_release( e.get() );
> }
>
>
Nice! If these produce and consume are the only two operations on the
array, from a cursory look, it should work fine. For the producer, a simple
store with release semantics might actually be enough.

The reason these operations should work is that they are actually move
operations and do not change the count itself.

What you can't do is copy a value out of the array leaving the original in
place.

-- gpd