In which cases is Arc<Mutex<T>> not the best way for sharing data across threads in Rust?

Question

From what I've learned, I should always choose Arc<T> for shared read access across threads and Arc<Mutex<T>> for shared write access across threads. Are there cases where I don't want to use Arc<T>/Arc<Mutex<T>> and instead do something completely different? E.g. do something like this:

unsafe impl Sync for MyStruct {}
unsafe impl Send for MyStruct {}

let shared_data_for_writing = Arc::from(MyStruct::new());

Answer 1

Sharing across threads

Besides Arc<T>, we can share objects across threads using scoped threads, e.g. by using crossbeam::scope and Scope::spawn. Scoped threads allow us to send borrowed pointers (&'a T) to threads spawned in a scope. The scope guarantees that the thread will terminate before the referent is dropped. Borrowed pointers have no runtime overhead compared to Arc<T> (Arc<T> takes a bit more memory and needs to maintain a reference counter using atomic instructions).

Mutating across threads

Mutex<T> is the most basic general-purpose wrapper for ensuring at most one thread may mutate a value at any given time. Mutex<T> has one drawback: if there are many threads that only want to read the value in the mutex, they can't do so concurrently, even though it would be safe. RwLock<T> solves this by allowing multiple concurrent readers (while still ensuring a writer has exclusive access).

Atomic types such as AtomicUsize also allow mutation across threads, but only for small values (8, 16, 32 or 64 bits ­– some processors support atomic operations on 128-bit values, but that's not exposed in the standard library yet; see atomic::Atomic for that). For example, instead of Arc<Mutex<usize>>, you could use Arc<AtomicUsize>. Atomic types do not require locking, but they are manipulated through atomic machine instructions. The set of atomic instructions is a bit different from the set of non-atomic instructions, so switching from a non-atomic type to an atomic type might not always be a "drop-in replacement".