[PATCH v3 4/4] rust: task: add `Send` marker to `Task`

[Alice Ryhl]

When a type also implements `Sync`, the meaning of `Send` is just "this
type may be accessed mutably from threads other than the one it is
created on". That's ok for this type.

Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
Reviewed-by: Andreas Hindborg <a.hindborg@xxxxxxxxxxx>
Reviewed-by: Gary Guo <gary@xxxxxxxxxxx>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@xxxxxxxxx>
Reviewed-by: Benno Lossin <benno.lossin@xxxxxxxxx>
---
 rust/kernel/task.rs | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
index 526d29a0ae27..7eda15e5f1b3 100644
--- a/rust/kernel/task.rs
+++ b/rust/kernel/task.rs
@@ -64,8 +64,14 @@ macro_rules! current {
 #[repr(transparent)]
 pub struct Task(pub(crate) Opaque<bindings::task_struct>);
 
-// SAFETY: It's OK to access `Task` through references from other threads because we're either
-// accessing properties that don't change (e.g., `pid`, `group_leader`) or that are properly
+// SAFETY: By design, the only way to access a `Task` is via the `current` function or via an
+// `ARef<Task>` obtained through the `AlwaysRefCounted` impl. This means that the only situation in
+// which a `Task` can be accessed mutably is when the refcount drops to zero and the destructor
+// runs. It is safe for that to happen on any thread, so it is ok for this type to be `Send`.
+unsafe impl Send for Task {}
+
+// SAFETY: It's OK to access `Task` through shared references from other threads because we're
+// either accessing properties that don't change (e.g., `pid`, `group_leader`) or that are properly
 // synchronised by C code (e.g., `signal_pending`).
 unsafe impl Sync for Task {}
 
-- 
2.41.0.rc0.172.g3f132b7071-goog