pythongh-129752: Don't update adaptive counters when TLBC=0 in free-threading. (pythongh-155497)

Author: corona10

Include/internal/pycore_backoff.h

@@ -108,6 +108,12 @@ backoff_counter_triggers(_Py_BackoffCounter counter)
     return counter.value_and_backoff < UNREACHABLE_BACKOFF;
 }
 
+static inline bool
+backoff_counter_is_unreachable(_Py_BackoffCounter counter)
+{
+    return (counter.value_and_backoff & BACKOFF_MASK) == UNREACHABLE_BACKOFF;
+}
+
 static inline _Py_BackoffCounter
 trigger_backoff_counter(void)
 {

Lib/test/test_thread_local_bytecode.py

@@ -108,7 +108,6 @@ def f(a, b, q=None):
         """)
         assert_python_ok("-X", "tlbc=1", "-c", code)
 
-    @support.skip_if_sanitizer("gh-129752: data race on adaptive counter", thread=True)
     def test_no_copies_if_tlbc_disabled(self):
         code = textwrap.dedent("""
         import queue

Misc/NEWS.d/next/Core_and_Builtins/2026-08-11-01-45-24.gh-issue-129752.Zo3OUx.rst

@@ -0,0 +1,2 @@
+Don't update adaptive counters in the free-threaded build when thread-local
+bytecode is disabled (``-X tlbc=0``). Patch by Donghee Na.

Python/ceval_macros.h

@@ -354,6 +354,25 @@ static void dtrace_function_return(_PyInterpreterFrame *);
 #define ADAPTIVE_COUNTER_TRIGGERS(COUNTER) \
     backoff_counter_triggers(forge_backoff_counter((COUNTER)))
 
+#ifdef Py_GIL_DISABLED
+/* Counters are unreachable when thread-local bytecode is disabled,
+ * so there is no need to update them. */
+#define ADVANCE_ADAPTIVE_COUNTER(COUNTER) \
+    do { \
+        _Py_BackoffCounter cnt = (COUNTER); \
+        if (!backoff_counter_is_unreachable(cnt)) { \
+            (COUNTER) = advance_backoff_counter(cnt); \
+        } \
+    } while (0);
+
+#define PAUSE_ADAPTIVE_COUNTER(COUNTER) \
+    do { \
+        _Py_BackoffCounter cnt = (COUNTER); \
+        if (!backoff_counter_is_unreachable(cnt)) { \
+            (COUNTER) = pause_backoff_counter(cnt); \
+        } \
+    } while (0);
+#else
 #define ADVANCE_ADAPTIVE_COUNTER(COUNTER) \
     do { \
         (COUNTER) = advance_backoff_counter((COUNTER)); \
@@ -363,6 +382,7 @@ static void dtrace_function_return(_PyInterpreterFrame *);
     do { \
         (COUNTER) = pause_backoff_counter((COUNTER)); \
     } while (0);
+#endif
 
 #ifdef ENABLE_SPECIALIZATION
 /* Multiple threads may execute these concurrently if thread-local bytecode is