[RFC v2 09/13] rust: sync: atomic: Add Atomic<*mut T>

[Boqun Feng <boqun.feng@gmail.com>]

Add atomic support for raw pointer values, similar to `isize` and
`usize`, the representation type is selected based on CONFIG_64BIT.

`*mut T` is not `Send`, however `Atomic<*mut T>` definitely needs to be
a `Sync`, and that's the whole point of atomics: being able to have
multiple shared references in different threads so that they can sync
with each other. As a result, a pointer value will be transferred from
one thread to another via `Atomic<*mut T>`:

	<thread 1>		<thread 2>
	x.store(p1, Relaxed);
				let p = x.load(p1, Relaxed);

This means a raw pointer value (`*mut T`) needs to be able to transfer
across thread boundaries, which is essentially `Send`.

To reflect this in the type system, and based on the fact that pointer
values can be transferred safely (only using them to dereference is
unsafe), as suggested by Alice, extend the `AllowAtomic` trait to
include a customized `Send` semantics, that is: `impl AllowAtomic` has
to be safe to be transferred across thread boundaries.

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
 rust/kernel/sync/atomic.rs         | 24 ++++++++++++++++++++++++
 rust/kernel/sync/atomic/generic.rs | 16 +++++++++++++---
 2 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
index 4166ad48604f..e62c3cd1d3ca 100644
--- a/rust/kernel/sync/atomic.rs
+++ b/rust/kernel/sync/atomic.rs
@@ -173,3 +173,27 @@ fn delta_into_repr(d: Self::Delta) -> Self::Repr {
         d as _
     }
 }
+
+/// ```rust
+/// use kernel::sync::atomic::{Atomic, Relaxed};
+///
+/// let x = Atomic::new(core::ptr::null_mut::<i32>());
+///
+/// assert!(x.load(Relaxed).is_null());
+/// ```
+// SAFETY: A `*mut T` has the same size and the alignment as `i64` for 64bit and the same as `i32`
+// for 32bit. And it's safe to transfer the ownership of a pointer value to another thread.
+unsafe impl<T> generic::AllowAtomic for *mut T {
+    #[cfg(CONFIG_64BIT)]
+    type Repr = i64;
+    #[cfg(not(CONFIG_64BIT))]
+    type Repr = i32;
+
+    fn into_repr(self) -> Self::Repr {
+        self as _
+    }
+
+    fn from_repr(repr: Self::Repr) -> Self {
+        repr as _
+    }
+}
diff --git a/rust/kernel/sync/atomic/generic.rs b/rust/kernel/sync/atomic/generic.rs
index a75c3e9f4c89..cff98469ed35 100644
--- a/rust/kernel/sync/atomic/generic.rs
+++ b/rust/kernel/sync/atomic/generic.rs
@@ -19,6 +19,10 @@
 #[repr(transparent)]
 pub struct Atomic<T: AllowAtomic>(Opaque<T>);
 
+// SAFETY: `Atomic<T>` is safe to send between execution contexts, because `T` is `AllowAtomic` and
+// `AllowAtomic`'s safety requirement guarantees that.
+unsafe impl<T: AllowAtomic> Send for Atomic<T> {}
+
 // SAFETY: `Atomic<T>` is safe to share among execution contexts because all accesses are atomic.
 unsafe impl<T: AllowAtomic> Sync for Atomic<T> {}
 
@@ -30,8 +34,13 @@ unsafe impl<T: AllowAtomic> Sync for Atomic<T> {}
 ///
 /// # Safety
 ///
-/// [`Self`] must have the same size and alignment as [`Self::Repr`].
-pub unsafe trait AllowAtomic: Sized + Send + Copy {
+/// - [`Self`] must have the same size and alignment as [`Self::Repr`].
+/// - The implementer must guarantee it's safe to transfer ownership from one execution context to
+///   another, this means it has to be a [`Send`], but because `*mut T` is not [`Send`] and that's
+///   the basic type needs to support atomic operations, so this safety requirement is added to
+///   [`AllowAtomic`] trait. This safety requirement is automatically satisfied if the type is a
+///   [`Send`].
+pub unsafe trait AllowAtomic: Sized + Copy {
     /// The backing atomic implementation type.
     type Repr: AtomicImpl;
 
@@ -42,7 +51,8 @@ pub unsafe trait AllowAtomic: Sized + Send + Copy {
     fn from_repr(repr: Self::Repr) -> Self;
 }
 
-// SAFETY: `T::Repr` is `Self` (i.e. `T`), so they have the same size and alignment.
+// SAFETY: `T::Repr` is `Self` (i.e. `T`), so they have the same size and alignment. And all
+// `AtomicImpl` types are `Send`.
 unsafe impl<T: AtomicImpl> AllowAtomic for T {
     type Repr = Self;
 
-- 
2.45.2