The KMS bindings ( https://patchwork.freedesktop.org/series/131522/ ) have
a few bindings that require manually acquiring specific locks before
calling certain functions. At the moment though, the only way of acquiring
these locks in bindings is to simply call the C locking functions directly
- since said locks are not acquired on the rust side of things.

However - if we add `#[repr(C)]` to `Lock<(), B>`, then given `()` is a ZST
- `Lock<(), B>` becomes equivalent in data layout to its inner `B::State`
type. Since locks in C don't have data explicitly associated with them
anyway, we can take advantage of this to add a `Lock::from_raw()` function
that can translate a raw pointer to `B::State` into its proper `Lock<(),
B>` equivalent. This lets us simply acquire a reference to the lock in
question and work with it like it was initialized on the rust side of
things, allowing us to use less unsafe code to implement bindings with lock
requirements.

Signed-off-by: Lyude Paul <lyude@redhat.com>

---

V2:
* Don't implement this for all ZST types, just implement it for ()
V3:
* Get rid of some unused imports
* Make sure that we run rustfmt
V4:
* Documentation fixes from Boqun
* Reword commit message (Boqun)

Signed-off-by: Lyude Paul <lyude@redhat.com>
---
 rust/kernel/sync/lock.rs | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
index 6d3c8874eb26a..ea65f84b76f8e 100644
--- a/rust/kernel/sync/lock.rs
+++ b/rust/kernel/sync/lock.rs
@@ -91,6 +91,7 @@ unsafe fn relock(ptr: *mut Self::State, guard_state: &mut Self::GuardState) {
 ///
 /// Exposes one of the kernel locking primitives. Which one is exposed depends on the lock
 /// [`Backend`] specified as the generic parameter `B`.
+#[repr(C)]
 #[pin_data]
 pub struct Lock<T: ?Sized, B: Backend> {
     /// The kernel lock object.
@@ -129,6 +130,28 @@ pub fn new(t: T, name: &'static CStr, key: &'static LockClassKey) -> impl PinIni
     }
 }
 
+impl<B: Backend> Lock<(), B> {
+    /// Constructs a [`Lock`] from a raw pointer.
+    ///
+    /// This can be useful for interacting with a lock which was initialised outside of Rust.
+    ///
+    /// # Safety
+    ///
+    /// The caller promises that `ptr` points to a valid initialised instance of [`State`] during
+    /// the whole lifetime of `'a`.
+    ///
+    /// [`State`]: Backend::State
+    pub unsafe fn from_raw<'a>(ptr: *mut B::State) -> &'a Self {
+        // SAFETY:
+        // - By the safety contract `ptr` must point to a valid initialised instance of `B::State`
+        // - Since the lock data type is `()` which is a ZST, `state` is the only non-ZST member of
+        //   the struct
+        // - Combined with `#[repr(C)]`, this guarantees `Self` has an equivalent data layout to
+        //   `B::State`.
+        unsafe { &*ptr.cast() }
+    }
+}
+
 impl<T: ?Sized, B: Backend> Lock<T, B> {
     /// Acquires the lock and gives the caller access to the data protected by it.
     pub fn lock(&self) -> Guard<'_, T, B> {
-- 
2.47.0

On Wed, Nov 20, 2024 at 12:11 AM Lyude Paul <lyude@redhat.com> wrote:
>
> The KMS bindings ( https://patchwork.freedesktop.org/series/131522/ ) have
> a few bindings that require manually acquiring specific locks before
> calling certain functions. At the moment though, the only way of acquiring
> these locks in bindings is to simply call the C locking functions directly
> - since said locks are not acquired on the rust side of things.
>
> However - if we add `#[repr(C)]` to `Lock<(), B>`, then given `()` is a ZST
> - `Lock<(), B>` becomes equivalent in data layout to its inner `B::State`
> type. Since locks in C don't have data explicitly associated with them
> anyway, we can take advantage of this to add a `Lock::from_raw()` function
> that can translate a raw pointer to `B::State` into its proper `Lock<(),
> B>` equivalent. This lets us simply acquire a reference to the lock in
> question and work with it like it was initialized on the rust side of
> things, allowing us to use less unsafe code to implement bindings with lock
> requirements.
>
> Signed-off-by: Lyude Paul <lyude@redhat.com>

Reviewed-by: Alice Ryhl <aliceryhl@google.com>