[PATCH] rust: sync: fix incorrect Sync bounds for LockedBy

[Alice Ryhl]

The `impl Sync for LockedBy` implementation has insufficient trait
bounds, as it only requires `T: Send`. However, `T: Sync` is also
required for soundness because the `LockedBy::access` method could be
used to provide shared access to the inner value from several threads in
parallel.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 7b1f55e3a984 ("rust: sync: introduce `LockedBy`")
Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
---
 rust/kernel/sync/locked_by.rs | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/rust/kernel/sync/locked_by.rs b/rust/kernel/sync/locked_by.rs
index babc731bd5f6..153ba4edcb03 100644
--- a/rust/kernel/sync/locked_by.rs
+++ b/rust/kernel/sync/locked_by.rs
@@ -83,9 +83,10 @@ pub struct LockedBy<T: ?Sized, U: ?Sized> {
 // SAFETY: `LockedBy` can be transferred across thread boundaries iff the data it protects can.
 unsafe impl<T: ?Sized + Send, U: ?Sized> Send for LockedBy<T, U> {}
 
-// SAFETY: `LockedBy` serialises the interior mutability it provides, so it is `Sync` as long as the
-// data it protects is `Send`.
-unsafe impl<T: ?Sized + Send, U: ?Sized> Sync for LockedBy<T, U> {}
+// SAFETY: Shared access to the `LockedBy` can provide both `&mut T` references in a synchronized
+// manner, or `&T` access in an unsynchronized manner. The `Send` trait is sufficient for the first
+// case, and `Sync` is sufficient for the second case.
+unsafe impl<T: ?Sized + Send + Sync, U: ?Sized> Sync for LockedBy<T, U> {}
 
 impl<T, U> LockedBy<T, U> {
     /// Constructs a new instance of [`LockedBy`].
@@ -127,7 +128,7 @@ pub fn access<'a>(&'a self, owner: &'a U) -> &'a T {
             panic!("mismatched owners");
         }
 
-        // SAFETY: `owner` is evidence that the owner is locked.
+        // SAFETY: `owner` is evidence that there are only shared references to the owner.
         unsafe { &*self.data.get() }
     }
 

---
base-commit: 93dc3be19450447a3a7090bd1dfb9f3daac3e8d2
change-id: 20240912-locked-by-sync-fix-07193df52f98

Best regards,
-- 
Alice Ryhl <aliceryhl@xxxxxxxxxx>