Question

I’m currently looking at a copy-on-write set implementation and want to confirm it’s thread safe. I’m fairly sure the only way it might not be is if the compiler is allowed to reorder statements within certain methods. For example, the Remove method looks like:

public bool Remove(T item)
{
    var newHashSet = new HashSet<T>(hashSet);
    var removed = newHashSet.Remove(item);
    hashSet = newHashSet;
    return removed;
}

Where hashSet is defined as

private volatile HashSet<T> hashSet;

So my question is, given that hashSet is volatile does it mean that the Remove on the new set happens before the write to the member variable? If not, then other threads may see the set before the remove has occurred.

I haven’t actually seen any issues with this in production, but I just want to confirm it is guaranteed to be safe.

UPDATE

To be more specific, there’s another method to get an IEnumerator:

public IEnumerator<T> GetEnumerator()
{
    return hashSet.GetEnumerator();
}

So the more specific question is: is there a guarantee that the returned IEnumerator will never throw a ConcurrentModificationException from a remove?

UPDATE 2

Sorry, the answers are all addressing the thread safety from multiple writers. Good points are raised, but that’s not what I’m trying to find out here. I’d like to know if the compiler is allowed to re-order the operations in Remove to something like this:

    var newHashSet = new HashSet<T>(hashSet);
    hashSet = newHashSet;                  // swapped
    var removed = newHashSet.Remove(item); // swapped
    return removed;

If this was possible, it would mean that a thread could call GetEnumerator after hashSet had been assigned, but before item was removed, which could lead to the collection being modified during enumeration.

Joe Duffy has a blog article that states:

Volatile on loads means ACQUIRE, no more, no less. (There are
additional compiler optimization restrictions, of course, like not
allowing hoisting outside of loops, but let’s focus on the MM aspects
for now.) The standard definition of ACQUIRE is that subsequent
memory operations may not move before the ACQUIRE instruction; e.g.
given { ld.acq X, ld Y }, the ld Y cannot occur before ld.acq X.
However, previous memory operations can certainly move after it; e.g.
given { ld X, ld.acq Y }, the ld.acq Y can indeed occur before the ld
X. The only processor Microsoft .NET code currently runs on for which
this actually occurs is IA64, but this is a notable area where CLR’s
MM is weaker than most machines. Next, all stores on .NET are RELEASE
(regardless of volatile, i.e. volatile is a no-op in terms of jitted
code). The standard definition of RELEASE is that previous memory
operations may not move after a RELEASE operation; e.g. given { st X,
st.rel Y }, the st.rel Y cannot occur before st X. However,
subsequent memory operations can indeed move before it; e.g. given {
st.rel X, ld Y }, the ld Y can move before st.rel X.

The way I read this is that the call to newHashSet.Remove requires a ld newHashSet and the write to hashSet requires a st.rel newHashSet. From the above definition of RELEASE no loads can move after the store RELEASE, so the statements cannot be reordered! Could someone confirm please confirm my interpretation is correct?

Answer 1

EDITED:

Thanks for clarifying the presence of an external lock for calls to Remove (and other collection mutations).

Because of RELEASE semantics you will not end up storing a new value to hashSet until after the value to the variable removed has been assigned (because st removed can’t be moved after st.rel hashSet).

Therefore, the ‘snapshot’ behaviour of GetEnumerator will work as intended, at least with respect to Remove and other mutators implemented in a similar fashion.