rust: sync: Add lock::Backend::assert_is_held()

Since we've exposed Lock::from_raw() and Guard::new() publically, we
want to be able to make sure that we assert that a lock is actually held
when constructing a Guard for it to handle instances of unsafe
Guard::new() calls outside of our lock module.

Hence add a new method assert_is_held() to Backend, which uses lockdep
to check whether or not a lock has been acquired. When lockdep is
disabled, this has no overhead.

[Boqun: Resolve the conflicts with exposing Guard::new(), reword the
 commit log a bit and format "unsafe { <statement>; }" into "unsafe {
 <statement> }" for the consistency. ]

Signed-off-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20241125204139.656801-1-lyude@redhat.com

rust/helpers/mutex.c

@@ -12,3 +12,8 @@ void rust_helper___mutex_init(struct mutex *mutex, const char *name,
 {
 	__mutex_init(mutex, name, key);
 }
+
+void rust_helper_mutex_assert_is_held(struct mutex *mutex)
+{
+	lockdep_assert_held(mutex);
+}

rust/helpers/spinlock.c

@@ -30,3 +30,8 @@ int rust_helper_spin_trylock(spinlock_t *lock)
 {
 	return spin_trylock(lock);
 }
+
+void rust_helper_spin_assert_is_held(spinlock_t *lock)
+{
+	lockdep_assert_held(lock);
+}

rust/kernel/sync/lock.rs

@@ -90,6 +90,13 @@ unsafe fn relock(ptr: *mut Self::State, guard_state: &mut Self::GuardState) {
         // SAFETY: The safety requirements ensure that the lock is initialised.
         *guard_state = unsafe { Self::lock(ptr) };
     }
+
+    /// Asserts that the lock is held using lockdep.
+    ///
+    /// # Safety
+    ///
+    /// Callers must ensure that [`Backend::init`] has been previously called.
+    unsafe fn assert_is_held(ptr: *mut Self::State);
 }
 
 /// A mutual exclusion primitive.
@@ -235,6 +242,9 @@ impl<'a, T: ?Sized, B: Backend> Guard<'a, T, B> {
     ///
     /// The caller must ensure that it owns the lock.
     pub unsafe fn new(lock: &'a Lock<T, B>, state: B::GuardState) -> Self {
+        // SAFETY: The caller can only hold the lock if `Backend::init` has already been called.
+        unsafe { B::assert_is_held(lock.state.get()) };
+
         Self {
             lock,
             state,

rust/kernel/sync/lock/mutex.rs

@@ -134,4 +134,9 @@ unsafe fn try_lock(ptr: *mut Self::State) -> Option<Self::GuardState> {
             None
         }
     }
+
+    unsafe fn assert_is_held(ptr: *mut Self::State) {
+        // SAFETY: The `ptr` pointer is guaranteed to be valid and initialized before use.
+        unsafe { bindings::mutex_assert_is_held(ptr) }
+    }
 }

rust/kernel/sync/lock/spinlock.rs

@@ -133,4 +133,9 @@ unsafe fn try_lock(ptr: *mut Self::State) -> Option<Self::GuardState> {
             None
         }
     }
+
+    unsafe fn assert_is_held(ptr: *mut Self::State) {
+        // SAFETY: The `ptr` pointer is guaranteed to be valid and initialized before use.
+        unsafe { bindings::spin_assert_is_held(ptr) }
+    }
 }