rust: types: add NotThreadSafe

This introduces a new marker type for types that shouldn't be thread
safe. By adding a field of this type to a struct, it becomes non-Send
and non-Sync, which means that it cannot be accessed in any way from
threads other than the one it was created on.

This is useful for APIs that require globals such as `current` to remain
constant while the value exists.

We update two existing users in the Kernel to use this helper:

 * `Task::current()` - moving the return type of this value to a
   different thread would not be safe as you can no longer be guaranteed
   that the `current` pointer remains valid.
 * Lock guards. Mutexes and spinlocks should be unlocked on the same
   thread as where they were locked, so we enforce this using the Send
   trait.

There are also additional users in later patches of this patchset. See
[1] and [2] for the discussion that led to the introduction of this
patch.

Link: https://lore.kernel.org/all/nFDPJFnzE9Q5cqY7FwSMByRH2OAn_BpI4H53NQfWIlN6I2qfmAqnkp2wRqn0XjMO65OyZY4h6P4K2nAGKJpAOSzksYXaiAK_FoH_8QbgBI4=@proton.me/ [1]
Link: https://lore.kernel.org/all/nFDPJFnzE9Q5cqY7FwSMByRH2OAn_BpI4H53NQfWIlN6I2qfmAqnkp2wRqn0XjMO65OyZY4h6P4K2nAGKJpAOSzksYXaiAK_FoH_8QbgBI4=@proton.me/ [2]
Suggested-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Björn Roy Baron <bjorn3_gh@protonmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240915-alice-file-v10-1-88484f7a3dcf@google.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

rust/kernel/types.rs

@@ -532,3 +532,24 @@ unsafe impl AsBytes for str {}
 // does not have any uninitialized portions either.
 unsafe impl<T: AsBytes> AsBytes for [T] {}
 unsafe impl<T: AsBytes, const N: usize> AsBytes for [T; N] {}
+
+/// Zero-sized type to mark types not [`Send`].
+///
+/// Add this type as a field to your struct if your type should not be sent to a different task.
+/// Since [`Send`] is an auto trait, adding a single field that is `!Send` will ensure that the
+/// whole type is `!Send`.
+///
+/// If a type is `!Send` it is impossible to give control over an instance of the type to another
+/// task. This is useful to include in types that store or reference task-local information. A file
+/// descriptor is an example of such task-local information.
+///
+/// This type also makes the type `!Sync`, which prevents immutable access to the value from
+/// several threads in parallel.
+pub type NotThreadSafe = PhantomData<*mut ()>;
+
+/// Used to construct instances of type [`NotThreadSafe`] similar to how `PhantomData` is
+/// constructed.
+///
+/// [`NotThreadSafe`]: type@NotThreadSafe
+#[allow(non_upper_case_globals)]
+pub const NotThreadSafe: NotThreadSafe = PhantomData;