Question

I am writing intrusive shared pointer and I am using C++11 <atomic> facilities for reference counter. Here are the relevant fragments of my code:

//...
mutable std::atomic<unsigned> count;
//...

void
SharedObject::addReference() const
{
    std::atomic_fetch_add_explicit (&count, 1u,
        std::memory_order_consume);
}

void
SharedObject::removeReference() const
{
    bool destroy;

    destroy = std::atomic_fetch_sub_explicit (&count, 1u,
        std::memory_order_consume) == 1;

    if (destroy)
        delete this;
}

I have started with memory_order_acquire and memory_order_release first but then I convinced myself that memory_order_consume should be good enough. After further deliberation it seems to me that even memory_order_relaxed should work.

Now, the question is whether I can use memory_order_consume for the operations or could I use weaker ordering (memory_order_relaxed) or should I use stricter ordering?

Answer 1

void
SharedObject::addReference() const
{
    std::atomic_fetch_add_explicit (&count, 1u, std::memory_order_relaxed);
}

void
SharedObject::removeReference() const
{
    if ( std::atomic_fetch_sub_explicit (&count, 1u, std::memory_order_release) == 1 ) {
         std::atomic_thread_fence(boost::memory_order_acquire);
         delete this;
    }
}

You want to use atomic_thread_fence such that the delete is strictly after the fetch_sub. Reference

Quote from the linked text:

Increasing the reference counter can always be done with
memory_order_relaxed: New references to an object can only be formed
from an existing reference, and passing an existing reference from one
thread to another must already provide any required synchronization.

It is important to enforce any possible access to the object in one
thread (through an existing reference) to happen before deleting the
object in a different thread. This is achieved by a “release”
operation after dropping a reference (any access to the object through
this reference must obviously happened before), and an “acquire”
operation before deleting the object.

It would be possible to use memory_order_acq_rel for the fetch_sub
operation, but this results in unneeded “acquire” operations when the
reference counter does not yet reach zero and may impose a performance
penalty.