Why there need memory order limit on reference counter?

Question

In the example of boost::atomic, the unref function:

void intrusive_ptr_release(const X * x)
{
  if (x->refcount_.fetch_sub(1, boost::memory_order_release) == 1) {
    boost::atomic_thread_fence(boost::memory_order_acquire);
    delete x;
  }
}

1: the fetch_sub op is limited by memory_order_release, which prevents preceding operations to be reordered past the point. But what are the possible scenes that would have such phenomenon?

2: in addition of memory_order_release on the atomic op, why there is an additional memory_order_acquire before the deletion?

This one seems better related: stackoverflow.com/questions/10268737/… — firda
– firda, Commented Aug 14, 2014 at 9:21

MSalters · Accepted Answer · 2014-08-14 12:00:03Z

1

For the first question, it prevents any use of (*x) to be reordered after the fetch_sub (when the reference count could be 0 and use is therefore banned). Possible causes are CPU reordering or compiler reordering. The second question is just the mirror of the release; release protects stores and acquire protects loads.

It may seem that refcount_fetch_sub(1, memory_order_acq_rel) works as well, but that protects just the refcount.

answered Aug 14, 2014 at 12:00

MSalters

182k11 gold badges171 silver badges376 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

jiandingzhe Over a year ago

I know both compiler and CPU may reorder commands. However, in this case, the delete x command is dependent on the result of the fetch_sub. Why there are still possibility that things in delete x may be reordered prior to fetch_sub?

MSalters Over a year ago

In a single-threaded view, reads could be reordered before an unrelated write, i.e. the other members of x may already be read to prevent a pipeline stall if the branch succeeds. And that in turn may cause the deletion of an older version of x when another thread had updated x.

curiousguy Over a year ago

"but that protects just the refcount." What do you mean?

rdb Over a year ago

Surely memory_order_acq_rel doesn't just protect the refcount? Rather the separate barrier is used as an optimization, so that it only uses an acquire barrier if the reference count reaches 0?

Collectives™ on Stack Overflow

Why there need memory order limit on reference counter?

1 Answer 1

4 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related