diff --git a/stdlib/@tests/stubtest_allowlists/common.txt b/stdlib/@tests/stubtest_allowlists/common.txt
index e87ba193c..7bfa46508 100644
--- a/stdlib/@tests/stubtest_allowlists/common.txt
+++ b/stdlib/@tests/stubtest_allowlists/common.txt
@@ -43,18 +43,6 @@ ipaddress._BaseAddress.is_unspecified
 ipaddress._BaseAddress.max_prefixlen
 ipaddress._BaseAddress.packed
 ipaddress._BaseNetwork.max_prefixlen
-multiprocessing.JoinableQueue
-multiprocessing.Queue
-multiprocessing.SimpleQueue
-multiprocessing.queues.JoinableQueue.__init__
-multiprocessing.queues.Queue.__init__
-multiprocessing.queues.SimpleQueue.__init__
-multiprocessing.synchronize.Barrier.__init__
-multiprocessing.synchronize.Condition.acquire
-multiprocessing.synchronize.Condition.release
-multiprocessing.synchronize.SemLock.__init__
-multiprocessing.synchronize.SemLock.acquire
-multiprocessing.synchronize.SemLock.release
 numbers.Number.__hash__  # typeshed marks this as abstract but code just sets this as None
 optparse.Values.__getattr__  # Some attributes are set in __init__ using setattr
 pickle.Pickler.reducer_override  # implemented in C pickler
@@ -410,6 +398,13 @@ inspect.Signature.__init__
 inspect.Parameter.empty  # set as private marker _empty
 inspect.Signature.empty  # set as private marker _empty
 
+# At runtime, these are functions from multiprocessing.context._default_context.
+# Typeshed makes them classes instead, which matches CPython documentation.
+# This has been heavily discussed, see #4266 for the primary issue about it.
+multiprocessing.JoinableQueue
+multiprocessing.Queue
+multiprocessing.SimpleQueue
+
 # These multiprocessing proxy methods have *args, **kwargs signatures at runtime,
 # But have more precise (accurate) signatures in the stub
 multiprocessing.managers.BaseListProxy.__imul__
@@ -432,6 +427,22 @@ multiprocessing.reduction.AbstractReducer.ForkingPickler
 # Non-private parameter on __del__
 multiprocessing.pool.Pool.__del__
 
+# These are because the ctx argument has a default value in the stubs but not
+# at runtime. This is a compromise between the runtime signatures of (for example)
+# multiprocessing.Queue and multiprocessing.queues.Queue, which typeshed
+# treats as the same object.
+multiprocessing.queues.JoinableQueue.__init__
+multiprocessing.queues.Queue.__init__
+multiprocessing.queues.SimpleQueue.__init__
+
+# These methods are dynamically created after object initialization,
+# copied from a wrapped lock object. Stubtest doesn't think they exist
+# because of that.
+multiprocessing.synchronize.Condition.acquire
+multiprocessing.synchronize.Condition.release
+multiprocessing.synchronize.SemLock.acquire
+multiprocessing.synchronize.SemLock.release
+
 # C signature is broader than what is actually accepted
 _?queue.SimpleQueue.__init__
 
diff --git a/stdlib/multiprocessing/synchronize.pyi b/stdlib/multiprocessing/synchronize.pyi
index d6f46b527..e3cbfbc0e 100644
--- a/stdlib/multiprocessing/synchronize.pyi
+++ b/stdlib/multiprocessing/synchronize.pyi
@@ -10,7 +10,7 @@ _LockLike: TypeAlias = Lock | RLock
 
 class Barrier(threading.Barrier):
     def __init__(
-        self, parties: int, action: Callable[[], object] | None = None, timeout: float | None = None, *ctx: BaseContext
+        self, parties: int, action: Callable[[], object] | None = None, timeout: float | None = None, *, ctx: BaseContext
     ) -> None: ...
 
 class Condition:
@@ -19,12 +19,14 @@ class Condition:
     def notify_all(self) -> None: ...
     def wait(self, timeout: float | None = None) -> bool: ...
     def wait_for(self, predicate: Callable[[], bool], timeout: float | None = None) -> bool: ...
-    def acquire(self, block: bool = ..., timeout: float | None = ...) -> bool: ...
-    def release(self) -> None: ...
     def __enter__(self) -> bool: ...
     def __exit__(
         self, exc_type: type[BaseException] | None, exc_val: BaseException | None, exc_tb: TracebackType | None, /
     ) -> None: ...
+    # These methods are copied from the lock passed to the constructor, or an
+    # instance of ctx.RLock() if lock was None.
+    def acquire(self, block: bool = True, timeout: float | None = None) -> bool: ...
+    def release(self) -> None: ...
 
 class Event:
     def __init__(self, *, ctx: BaseContext) -> None: ...
@@ -35,12 +37,14 @@ class Event:
 
 # Not part of public API
 class SemLock:
-    def acquire(self, block: bool = ..., timeout: float | None = ...) -> bool: ...
-    def release(self) -> None: ...
+    def __init__(self, kind: int, value: int, maxvalue: int, *, ctx: BaseContext | None) -> None: ...
     def __enter__(self) -> bool: ...
     def __exit__(
         self, exc_type: type[BaseException] | None, exc_val: BaseException | None, exc_tb: TracebackType | None, /
     ) -> None: ...
+    # These methods are copied from the wrapped _multiprocessing.SemLock object
+    def acquire(self, block: bool = True, timeout: float | None = None) -> bool: ...
+    def release(self) -> None: ...
 
 class Lock(SemLock):
     def __init__(self, *, ctx: BaseContext) -> None: ...