From a8ee30f45d5d57467ddb7877ed6914d0eba0af7f Mon Sep 17 00:00:00 2001 From: Alice Ryhl Date: Sun, 15 Sep 2024 14:41:28 +0000 Subject: [PATCH] rust: sync: require `T: Sync` for `LockedBy::access` 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@vger.kernel.org Fixes: 7b1f55e3a984 ("rust: sync: introduce `LockedBy`") Signed-off-by: Alice Ryhl Suggested-by: Boqun Feng Reviewed-by: Gary Guo Link: https://lore.kernel.org/r/20240915-locked-by-sync-fix-v2-1-1a8d89710392@google.com Signed-off-by: Miguel Ojeda --- 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 { // SAFETY: `LockedBy` can be transferred across thread boundaries iff the data it protects can. unsafe impl Send for LockedBy {} -// 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 Sync for LockedBy {} impl LockedBy { @@ -118,7 +122,10 @@ impl LockedBy { /// /// 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::() > 0, "`U` cannot be a ZST because `owner` wouldn't be unique" @@ -127,7 +134,10 @@ impl LockedBy { 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() } }