mlodato517 / rust_atomics_and_locks Goto Github PK

This reasoning:

is incorrect. The book mentions:

As a precaution, UnsafeCell does not implement Sync, which means that our type is now no longer shareable between threads, making it rather useless. To fix that, we need to promise to the compiler that it is actually safe for our type to be shared between threads. However, since the lock can be used to send values of type T from one thread to another, we must limit this promise to types that are safe to send between threads. So, we (unsafely) implement Sync for SpinLock for all T that implement Send, like this:

unsafe impl<T> Sync for SpinLock<T> where T: Send {}

Note that we don’t need to require that T is Sync, because our SpinLock will only allow one thread at a time to access the T it protects. Only if we were to give multiple threads access at once, like a reader-writer lock does for readers, would we (additionally) need to require T: Sync.