gh-150818: Speed up logging.getLogger() for existing loggers (GH-150825)

(cherry picked from commit e833f57)

Co-authored-by: Bernát Gábor <gaborjbernat@gmail.com>

Author: gaborbernat

Lib/logging/__init__.py

@@ -1373,9 +1373,16 @@ def getLogger(self, name):
         logger and fix up the parent/child references which pointed to the
         placeholder to now point to the logger.
         """
-        rv = None
         if not isinstance(name, str):
             raise TypeError('A logger name must be a string')
+        # Fast path: an already-registered, non-placeholder logger can be
+        # returned without taking the lock. dict.get() is atomic under both
+        # the GIL and free threading, and a Logger is fully initialised before
+        # being inserted into loggerDict under the lock, so this never sees a
+        # partially-constructed object.
+        rv = self.loggerDict.get(name)
+        if rv is not None and not isinstance(rv, PlaceHolder):
+            return rv
         with _lock:
             if name in self.loggerDict:
                 rv = self.loggerDict[name]

Misc/NEWS.d/next/Library/2026-06-02-15-44-58.gh-issue-150818.Orcefu.rst

@@ -0,0 +1,3 @@
+Speed up :func:`logging.getLogger` with a lock-free fast path that returns an
+already-registered logger without acquiring the logging lock. Patch by Bernát
+Gábor.