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

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
new file mode 100644 (file)
index 0000000..ce29dde
--- /dev/null
+++ 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
index b125ad0c9e356d50ef0fd219912f279bacc83179..ea6ac00bfeccb4e3447f20e61d456278c56f5210 100644 (file)
--- 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;
     }