Concurrent Max

Can you think of a better way to implement this code?

    private volatile Guid lastEtag;
    private readonly object lastEtagLocker = new object();

    internal void UpdateLastWrittenEtag(Guid? etag)
    {
        if (etag == null)
            return;

        var newEtag = etag.Value.ToByteArray();

        // not the most recent etag
        if (Buffers.Compare(lastEtag.ToByteArray(), newEtag) <= 0)
        {
            return;
        }

        lock (lastEtagLocker)
        {
            // not the most recent etag
            if (Buffers.Compare(lastEtag.ToByteArray(), newEtag) <= 0)
            {
                return;
            }

            lastEtag = etag.Value;
        }
    }

We have multiple threads calling this function, and we need to ensure that lastEtag value is always the maximum value. This has the potential to be called often, so I want to make sure that I chose the best way to do this. Ideas?