On 7/14/23 06:13, Asahi Lina wrote:
Using macros to create lock classes all over the place is unergonomic,
and makes it impossible to add new features that require lock classes to
code such as Arc<> without changing all callers.
Rust has the ability to track the caller's identity by file/line/column
number, and we can use that to dynamically generate lock classes
instead.
Signed-off-by: Asahi Lina <lina@xxxxxxxxxxxxx>
---
[...]
+
+const LOCK_CLASS_BUCKETS: usize = 1024;
+
+#[track_caller]
+fn caller_lock_class_inner() -> Result<&'static DynLockClassKey> {
+ // This is just a hack to make the below static array initialization work.
+ #[allow(clippy::declare_interior_mutable_const)]
+ const ATOMIC_PTR: AtomicPtr<Mutex<Vec<&'static DynLockClassKey>>> =
+ AtomicPtr::new(core::ptr::null_mut());
+
+ #[allow(clippy::complexity)]
+ static LOCK_CLASSES: [AtomicPtr<Mutex<Vec<&'static DynLockClassKey>>>; LOCK_CLASS_BUCKETS] =
+ [ATOMIC_PTR; LOCK_CLASS_BUCKETS];
+
+ let loc = core::panic::Location::caller();
+ let loc_key = LocationKey::new(loc);
+
+ let index = (loc_key.hash % (LOCK_CLASS_BUCKETS as u64)) as usize;
+ let slot = &LOCK_CLASSES[index];
+
+ let mut ptr = slot.load(Ordering::Relaxed);
+ if ptr.is_null() {
+ let new_element = Box::pin_init(new_mutex!(Vec::new()))?;
+
+ if let Err(e) = slot.compare_exchange(
+ core::ptr::null_mut(),
+ // SAFETY: We never move out of this Box
+ Box::into_raw(unsafe { Pin::into_inner_unchecked(new_element) }),
+ Ordering::Relaxed,
+ Ordering::Relaxed,
+ ) {
+ // SAFETY: We just got this pointer from `into_raw()`
+ unsafe { Box::from_raw(e) };
+ }
+
+ ptr = slot.load(Ordering::Relaxed);
+ assert!(!ptr.is_null());
+ }
+
+ // SAFETY: This mutex was either just created above or previously allocated,
+ // and we never free these objects so the pointer is guaranteed to be valid.
+ let mut guard = unsafe { (*ptr).lock() };
+
+ for i in guard.iter() {
+ if i.loc == loc_key {
+ return Ok(i);
+ }
+ }
+
+ // We immediately leak the class, so it becomes 'static
+ let new_class = Box::leak(Box::try_new(DynLockClassKey {
+ key: Opaque::zeroed(),
+ loc: loc_key,
+ name: CString::try_from_fmt(fmt!("{}:{}:{}", loc.file(), loc.line(), loc.column()))?,
+ })?);
+
+ // SAFETY: This is safe to call with a pointer to a dynamically allocated lockdep key,
+ // and we never free the objects so it is safe to never unregister the key.
+ unsafe { bindings::lockdep_register_key(new_class.key.get()) };
+
+ guard.try_push(new_class)?;
+
+ Ok(new_class)
+}
+
[...]
Is there any problem if we have many `DynLockClassKey`s leaked or not?
