The `LockedBy::access` method only requires a shared reference to the
owner, so if we have shared access to the `LockedBy` from several
threads at once, then two threads could call `access` in parallel and
both obtain a shared reference to the inner value. Thus, require that
`T: Sync` when calling the `access` method.
An alternative is to require `T: Sync` in the `impl Sync for LockedBy`.
This patch does not choose that approach as it gives up the ability to
use `LockedBy` with `!Sync` types, which is okay as long as you only use
`access_mut`.
Cc: stable@xxxxxxxxxxxxxxx
Fixes: 7b1f55e3a984 ("rust: sync: introduce `LockedBy`")
Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
---
Changes in v2:
- Use a `where T: Sync` on `access` instead of changing `impl Sync for
LockedBy`.
- Link to v1: https://lore.kernel.org/r/20240912-locked-by-sync-fix-v1-1-26433cbccbd2@xxxxxxxxxx
---
rust/kernel/sync/locked_by.rs | 18 ++++++++++++++----
1 file changed, 14 insertions(+), 4 deletions(-)
diff --git a/rust/kernel/sync/locked_by.rs b/rust/kernel/sync/locked_by.rs
index babc731bd5f6..ce2ee8d87865 100644
--- a/rust/kernel/sync/locked_by.rs
+++ b/rust/kernel/sync/locked_by.rs
@@ -83,8 +83,12 @@ 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`.
+// SAFETY: If `T` is not `Sync`, then parallel shared access to this `LockedBy` allows you to use
+// `access_mut` to hand out `&mut T` on one thread at the time. The requirement that `T: Send` is
+// sufficient to allow that.
+//
+// If `T` is `Sync`, then the `access` method also becomes available, which allows you to obtain
+// several `&T` from several threads at once. However, this is okay as `T` is `Sync`.
unsafe impl<T: ?Sized + Send, U: ?Sized> Sync for LockedBy<T, U> {}
impl<T, U> LockedBy<T, U> {
@@ -118,7 +122,10 @@ impl<T: ?Sized, U> LockedBy<T, U> {
///
/// Panics if `owner` is different from the data protected by the lock used in
/// [`new`](LockedBy::new).
- pub fn access<'a>(&'a self, owner: &'a U) -> &'a T {
+ pub fn access<'a>(&'a self, owner: &'a U) -> &'a T
+ where
+ T: Sync,
+ {
build_assert!(
size_of::<U>() > 0,
"`U` cannot be a ZST because `owner` wouldn't be unique"
@@ -127,7 +134,10 @@ 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 for the
+ // duration of 'a, so it's not possible to use `Self::access_mut` to obtain a mutable
+ // reference to the inner value that aliases with this shared reference. The type is `Sync`
+ // so there are no other requirements.
unsafe { &*self.data.get() }
}
---
base-commit: 93dc3be19450447a3a7090bd1dfb9f3daac3e8d2
change-id: 20240912-locked-by-sync-fix-07193df52f98
Best regards,
--
Alice Ryhl <aliceryhl@xxxxxxxxxx>
