How do I share data between several threads when I need it mutable only rarely?

I have some data of type T which implements neither Copy nor Clone.

1. If I want to share my data between several threads immutably, I'll use Arc<T>.
2. If I want to share it mutably, I'll use Arc<Mutex<T>>.

What if I want to share it first mutably, and then immutably in a loop? So:

• I cannot use either Arc<T> or Arc<Mutex<Arc<T>>> because I will not be able to mutate the data in the "mutable" threads.
• I can use Arc<Mutex<T>>, but then I have to lock() it in each of the "immutable" threads to reach T, losing parallelism.
• I cannot copy data after mutation is complete, because it is expensive (or there is no way to implement Clone).

What is the right effective solution then?

A std::sync::RwLock is what I am looking for, thanks @Shepmaster!