rust: lockdep: Use Pin for all LockClassKey usages

Reintroduce dynamically-allocated LockClassKeys such that they are
automatically (de)registered. Require that all usages of LockClassKeys
ensure that they are Pin'd.

Currently, only `'static` LockClassKeys are supported, so Pin is
redundant. However, it is intended that dynamically-allocated
LockClassKeys will eventually be supported, so using Pin from the outset
will make that change simpler.

Closes: Rust-for-Linux#1102
Suggested-by: Benno Lossin <benno.lossin@proton.me>
Suggested-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Mitchell Levy <levymitchell0@gmail.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20250207-rust-lockdep-v4-2-7a50a7e88656@gmail.com

rust/helpers/helpers.c

@@ -30,6 +30,7 @@
 #include "signal.c"
 #include "slab.c"
 #include "spinlock.c"
+#include "sync.c"
 #include "task.c"
 #include "uaccess.c"
 #include "vmalloc.c"

rust/helpers/sync.c

@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/lockdep.h>
+
+void rust_helper_lockdep_register_key(struct lock_class_key *k)
+{
+	lockdep_register_key(k);
+}
+
+void rust_helper_lockdep_unregister_key(struct lock_class_key *k)
+{
+	lockdep_unregister_key(k);
+}

rust/kernel/sync.rs

@@ -5,6 +5,8 @@
 //! This module contains the kernel APIs related to synchronisation that have been ported or
 //! wrapped for usage by Rust code in the kernel.
 
+use crate::pin_init;
+use crate::prelude::*;
 use crate::types::Opaque;
 
 mod arc;
@@ -23,15 +25,64 @@ pub use locked_by::LockedBy;
 
 /// Represents a lockdep class. It's a wrapper around C's `lock_class_key`.
 #[repr(transparent)]
-pub struct LockClassKey(Opaque<bindings::lock_class_key>);
+#[pin_data(PinnedDrop)]
+pub struct LockClassKey {
+    #[pin]
+    inner: Opaque<bindings::lock_class_key>,
+}
 
 // SAFETY: `bindings::lock_class_key` is designed to be used concurrently from multiple threads and
 // provides its own synchronization.
 unsafe impl Sync for LockClassKey {}
 
 impl LockClassKey {
+    /// Initializes a dynamically allocated lock class key. In the common case of using a
+    /// statically allocated lock class key, the static_lock_class! macro should be used instead.
+    ///
+    /// # Example
+    /// ```
+    /// # use kernel::{c_str, stack_pin_init};
+    /// # use kernel::alloc::KBox;
+    /// # use kernel::types::ForeignOwnable;
+    /// # use kernel::sync::{LockClassKey, SpinLock};
+    ///
+    /// let key = KBox::pin_init(LockClassKey::new_dynamic(), GFP_KERNEL)?;
+    /// let key_ptr = key.into_foreign();
+    ///
+    /// {
+    ///     stack_pin_init!(let num: SpinLock<u32> = SpinLock::new(
+    ///         0,
+    ///         c_str!("my_spinlock"),
+    ///         // SAFETY: `key_ptr` is returned by the above `into_foreign()`, whose
+    ///         // `from_foreign()` has not yet been called.
+    ///         unsafe { <Pin<KBox<LockClassKey>> as ForeignOwnable>::borrow(key_ptr) }
+    ///     ));
+    /// }
+    ///
+    /// // SAFETY: We dropped `num`, the only use of the key, so the result of the previous
+    /// // `borrow` has also been dropped. Thus, it's safe to use from_foreign.
+    /// unsafe { drop(<Pin<KBox<LockClassKey>> as ForeignOwnable>::from_foreign(key_ptr)) };
+    ///
+    /// # Ok::<(), Error>(())
+    /// ```
+    pub fn new_dynamic() -> impl PinInit<Self> {
+        pin_init!(Self {
+            // SAFETY: lockdep_register_key expects an uninitialized block of memory
+            inner <- Opaque::ffi_init(|slot| unsafe { bindings::lockdep_register_key(slot) })
+        })
+    }
+
     pub(crate) fn as_ptr(&self) -> *mut bindings::lock_class_key {
-        self.0.get()
+        self.inner.get()
+    }
+}
+
+#[pinned_drop]
+impl PinnedDrop for LockClassKey {
+    fn drop(self: Pin<&mut Self>) {
+        // SAFETY: self.as_ptr was registered with lockdep and self is pinned, so the address
+        // hasn't changed. Thus, it's safe to pass to unregister.
+        unsafe { bindings::lockdep_unregister_key(self.as_ptr()) }
     }
 }
 
@@ -44,7 +95,7 @@ macro_rules! static_lock_class {
             // SAFETY: lockdep expects uninitialized memory when it's handed a statically allocated
             // lock_class_key
             unsafe { ::core::mem::MaybeUninit::uninit().assume_init() };
-        &CLASS
+        $crate::prelude::Pin::static_ref(&CLASS)
     }};
 }
 

rust/kernel/sync/condvar.rs

@@ -17,8 +17,7 @@ use crate::{
     time::Jiffies,
     types::Opaque,
 };
-use core::marker::PhantomPinned;
-use core::ptr;
+use core::{marker::PhantomPinned, pin::Pin, ptr};
 use macros::pin_data;
 
 /// Creates a [`CondVar`] initialiser with the given name and a newly-created lock class.
@@ -103,7 +102,7 @@ unsafe impl Sync for CondVar {}
 
 impl CondVar {
     /// Constructs a new condvar initialiser.
-    pub fn new(name: &'static CStr, key: &'static LockClassKey) -> impl PinInit<Self> {
+    pub fn new(name: &'static CStr, key: Pin<&'static LockClassKey>) -> impl PinInit<Self> {
         pin_init!(Self {
             _pin: PhantomPinned,
             // SAFETY: `slot` is valid while the closure is called and both `name` and `key` have

rust/kernel/sync/lock.rs

@@ -12,7 +12,7 @@ use crate::{
     str::CStr,
     types::{NotThreadSafe, Opaque, ScopeGuard},
 };
-use core::{cell::UnsafeCell, marker::PhantomPinned};
+use core::{cell::UnsafeCell, marker::PhantomPinned, pin::Pin};
 use macros::pin_data;
 
 pub mod mutex;
@@ -129,7 +129,7 @@ unsafe impl<T: ?Sized + Send, B: Backend> Sync for Lock<T, B> {}
 
 impl<T, B: Backend> Lock<T, B> {
     /// Constructs a new lock initialiser.
-    pub fn new(t: T, name: &'static CStr, key: &'static LockClassKey) -> impl PinInit<Self> {
+    pub fn new(t: T, name: &'static CStr, key: Pin<&'static LockClassKey>) -> impl PinInit<Self> {
         pin_init!(Self {
             data: UnsafeCell::new(t),
             _pin: PhantomPinned,

rust/kernel/sync/lock/global.rs

@@ -13,6 +13,7 @@ use crate::{
 use core::{
     cell::UnsafeCell,
     marker::{PhantomData, PhantomPinned},
+    pin::Pin,
 };
 
 /// Trait implemented for marker types for global locks.
@@ -26,7 +27,7 @@ pub trait GlobalLockBackend {
     /// The backend used for this global lock.
     type Backend: Backend + 'static;
     /// The class for this global lock.
-    fn get_lock_class() -> &'static LockClassKey;
+    fn get_lock_class() -> Pin<&'static LockClassKey>;
 }
 
 /// Type used for global locks.
@@ -270,7 +271,7 @@ macro_rules! global_lock {
             type Item = $valuety;
             type Backend = $crate::global_lock_inner!(backend $kind);
 
-            fn get_lock_class() -> &'static $crate::sync::LockClassKey {
+            fn get_lock_class() -> Pin<&'static $crate::sync::LockClassKey> {
                 $crate::static_lock_class!()
             }
         }

rust/kernel/sync/poll.rs

@@ -89,7 +89,7 @@ pub struct PollCondVar {
 
 impl PollCondVar {
     /// Constructs a new condvar initialiser.
-    pub fn new(name: &'static CStr, key: &'static LockClassKey) -> impl PinInit<Self> {
+    pub fn new(name: &'static CStr, key: Pin<&'static LockClassKey>) -> impl PinInit<Self> {
         pin_init!(Self {
             inner <- CondVar::new(name, key),
         })

rust/kernel/workqueue.rs

@@ -369,7 +369,7 @@ unsafe impl<T: ?Sized, const ID: u64> Sync for Work<T, ID> {}
 impl<T: ?Sized, const ID: u64> Work<T, ID> {
     /// Creates a new instance of [`Work`].
     #[inline]
-    pub fn new(name: &'static CStr, key: &'static LockClassKey) -> impl PinInit<Self>
+    pub fn new(name: &'static CStr, key: Pin<&'static LockClassKey>) -> impl PinInit<Self>
     where
         T: WorkItem<ID>,
     {