[Question] What is purpose of Phantomdata in LockGuard? (api.rs)

[dubuduru]

As far I understood, phantomdata is for mimicking as if the type stores a value of type T.
Why do we need this in struct LockGuard? The type T is *const (), which is !Send + !Sync.
At first I thought it is to make LockGuard not Sendable and Syncable, but I'm not sure. Is it right?

[tomtomjhj]

At first I thought it is to make LockGuard not Sendable and Syncable, but I'm not sure. Is it right?

Yes. This enforces that the lock not be unlocked by threads other than the thread that acquired the lock.

[dubuduru]

Thanks. Then why do we implement Send or Sync for LockGuard?
In api.rs line 118-119:
unsafe impl<'s, L: RawLock, T: Send> Send for LockGuard<'s, L, T> {}
unsafe impl<'s, L: RawLock, T: Sync> Sync for LockGuard<'s, L, T> {}

[tomtomjhj]

hmm that's weird. Sync makes sense, but Send not in general. @jeehoonkang

[Lee-Janggun]

Doesn't Send make sense when T is Send? As I understand it, PhantomData is there to prevent LockGuard being derived as general Send and Sync, and the impl is there to make it available for special cases.

[Lee-Janggun]

Ok, I did some searching, and this is actually a pretty interesting problem.

• Rust standard: std::sync::MutexGuard is specifically marked as not Send. In Rust, this was decided as the inner pthread used did not allow locks to be sent across threads. Recently, Rust has improved its synchronization primitives to not depend on pthread on most platforms. Now does this suggest MutexGuard may become Send? Unfortunately, (1) the improved implementations may still be restricted to the same limitations (2) some platforms still depends on pthread, and (3) this it is a breaking change, hence this will most likely never become Send.
• Our situation: Then since we do not depend on any pthread, can we implement Send? The answer I think is yes. In parking_lock, we can see that it is indeed Send, which is possible because they implement their own locks. As we are also implement our own locks, I think having Send for our locks should be Ok.

[dubuduru]

Thank you so much! Now I see the point.