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

3 files changed, 20 insertions, 0 deletions

diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
index 72dbf3fbb259..eb80048e0110 100644
--- a/rust/kernel/sync/lock.rs
+++ b/rust/kernel/sync/lock.rs
@@ -90,6 +90,13 @@ pub unsafe trait Backend {
         // 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,
diff --git a/rust/kernel/sync/lock/mutex.rs b/rust/kernel/sync/lock/mutex.rs
index 10a70c07268d..70cadbc2e8e2 100644
--- a/rust/kernel/sync/lock/mutex.rs
+++ b/rust/kernel/sync/lock/mutex.rs
@@ -134,4 +134,9 @@ unsafe impl super::Backend for MutexBackend {
             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) }
+    }
 }
diff --git a/rust/kernel/sync/lock/spinlock.rs b/rust/kernel/sync/lock/spinlock.rs
index 081c0220013b..ab2f8d075311 100644
--- a/rust/kernel/sync/lock/spinlock.rs
+++ b/rust/kernel/sync/lock/spinlock.rs
@@ -133,4 +133,9 @@ unsafe impl super::Backend for SpinLockBackend {
             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) }
+    }
 }