Dropping

We now need a way to decrease the reference count and drop the data once it is low enough, otherwise the data will live forever on the heap.

To do this, we can implement Drop.

Basically, we need to:

Decrement the reference count
If there is only one reference remaining to the data, then:
Atomically fence the data to prevent reordering of the use and deletion of the data
Drop the inner data

First, we'll need to get access to the ArcInner:

let inner = unsafe { self.ptr.as_ref() };

Now, we need to decrement the reference count. To streamline our code, we can also return if the returned value from fetch_sub (the value of the reference count before decrementing it) is not equal to 1 (which happens when we are not the last reference to the data).

if inner.rc.fetch_sub(1, Ordering::Release) != 1 {
    return;
}

We then need to create an atomic fence to prevent reordering of the use of the data and deletion of the data. As described in the standard library's implementation of Arc:

This fence is needed to prevent reordering of use of the data and deletion of the data. Because it is marked Release, the decreasing of the reference count synchronizes with this Acquire fence. This means that use of the data happens before decreasing the reference count, which happens before this fence, which happens before the deletion of the data.

As explained in the Boost documentation,

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.

In particular, while the contents of an Arc are usually immutable, it's possible to have interior writes to something like a Mutex. Since a Mutex is not acquired when it is deleted, we can't rely on its synchronization logic to make writes in thread A visible to a destructor running in thread B.

Also note that the Acquire fence here could probably be replaced with an Acquire load, which could improve performance in highly-contended situations. See 2.

To do this, we do the following:

#![allow(unused)]
fn main() {
use std::sync::atomic::Ordering;
use std::sync::atomic;
atomic::fence(Ordering::Acquire);
}

Finally, we can drop the data itself. We use Box::from_raw to drop the boxed ArcInner<T> and its data. This takes a *mut T and not a NonNull<T>, so we must convert using NonNull::as_ptr.

unsafe { Box::from_raw(self.ptr.as_ptr()); }

This is safe as we know we have the last pointer to the ArcInner and that its pointer is valid.

Now, let's wrap this all up inside the Drop implementation:

impl<T> Drop for Arc<T> {
    fn drop(&mut self) {
        let inner = unsafe { self.ptr.as_ref() };
        if inner.rc.fetch_sub(1, Ordering::Release) != 1 {
            return;
        }
        // This fence is needed to prevent reordering of the use and deletion
        // of the data.
        atomic::fence(Ordering::Acquire);
        // This is safe as we know we have the last pointer to the `ArcInner`
        // and that its pointer is valid.
        unsafe { Box::from_raw(self.ptr.as_ptr()); }
    }
}

The Rustonomicon

Dropping