Align type cache seqlock writer protocol with CPython

youknowone · youknowone · commit de0109941436 · 2026-03-05T17:30:42.000+09:00
diff --git a/crates/compiler-core/src/bytecode.rs b/crates/compiler-core/src/bytecode.rs
@@ -623,6 +623,10 @@ impl CodeUnits {
     ///
     /// # Safety
     /// - `index` must be in bounds.
+    /// - `value` must be `0` or a valid `*const PyObject` encoded as `usize`.
+    /// - Callers must follow the cache invalidation/upgrade protocol:
+    ///   invalidate the version guard before writing and publish the new
+    ///   version after writing.
     pub unsafe fn write_cache_ptr(&self, index: usize, value: usize) {
         self.pointer_cache[index].store(value, Ordering::Relaxed);
     }
diff --git a/crates/vm/src/builtins/type.rs b/crates/vm/src/builtins/type.rs
@@ -104,7 +104,28 @@ impl TypeCacheEntry {
 
     #[inline]
     fn begin_write(&self) {
-        self.sequence.fetch_add(1, Ordering::AcqRel);
+        let mut seq = self.sequence.load(Ordering::Acquire);
+        loop {
+            while (seq & 1) != 0 {
+                core::hint::spin_loop();
+                seq = self.sequence.load(Ordering::Acquire);
+            }
+            match self.sequence.compare_exchange_weak(
+                seq,
+                seq.wrapping_add(1),
+                Ordering::AcqRel,
+                Ordering::Acquire,
+            ) {
+                Ok(_) => {
+                    core::sync::atomic::fence(Ordering::Release);
+                    break;
+                }
+                Err(observed) => {
+                    core::hint::spin_loop();
+                    seq = observed;
+                }
+            }
+        }
     }
 
     #[inline]
