Fix needless spinning in _PyMutex_LockTimed with zero timeout (gh-135872)

jtibbertsma · web-flow · commit cbfaf41caf13 · 2025-06-25T16:41:36.000Z
The free threading build could spin unnecessarily on `_Py_yield()` if the initial
compare and swap failed.
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-06-23-18-08-32.gh-issue-135871.50C528.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-06-23-18-08-32.gh-issue-135871.50C528.rst
@@ -0,0 +1,2 @@
+Non-blocking mutex lock attempts now return immediately when the lock is busy
+instead of briefly spinning in the :term:`free threading` build.
diff --git a/Python/lock.c b/Python/lock.c
@@ -58,7 +58,7 @@ _PyMutex_LockTimed(PyMutex *m, PyTime_t timeout, _PyLockFlags flags)
             return PY_LOCK_ACQUIRED;
         }
     }
-    else if (timeout == 0) {
+    if (timeout == 0) {
         return PY_LOCK_FAILURE;
     }
 

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,2 @@`
	`1`	`+Non-blocking mutex lock attempts now return immediately when the lock is busy`
	`2`	+instead of briefly spinning in the :term:`free threading` build.
Original file line number	Diff line number	Diff line change
`@@ -58,7 +58,7 @@ _PyMutex_LockTimed(PyMutex *m, PyTime_t timeout, _PyLockFlags flags)`
`58`	`58`	`return PY_LOCK_ACQUIRED;`
`59`	`59`	`}`
`60`	`60`	`}`
`61`		`- else if (timeout == 0) {`
	`61`	`+ if (timeout == 0) {`
`62`	`62`	`return PY_LOCK_FAILURE;`
`63`	`63`	`}`
`64`	`64`