diff --git a/trio/_highlevel_socket.py b/trio/_highlevel_socket.py
index 2889e23f13..28c8a7c08a 100644
--- a/trio/_highlevel_socket.py
+++ b/trio/_highlevel_socket.py
@@ -6,7 +6,7 @@
 from . import _core
 from . import socket as tsocket
 from ._socket import real_socket_type
-from ._util import UnLock
+from ._util import ConflictDetector
 from .abc import HalfCloseableStream, Listener
 from ._highlevel_generic import (
     ClosedStreamError, BrokenStreamError, ClosedListenerError
@@ -71,8 +71,7 @@ def __init__(self, socket):
             raise err from None
 
         self.socket = socket
-        self._send_lock = UnLock(
-            _core.ResourceBusyError,
+        self._send_conflict_detector = ConflictDetector(
             "another task is currently sending data on this SocketStream"
         )
 
@@ -105,19 +104,19 @@ async def send_all(self, data):
         if self.socket.did_shutdown_SHUT_WR:
             await _core.yield_briefly()
             raise ClosedStreamError("can't send data after sending EOF")
-        with self._send_lock.sync:
+        with self._send_conflict_detector.sync:
             with _translate_socket_errors_to_stream_errors():
                 await self.socket.sendall(data)
 
     async def wait_send_all_might_not_block(self):
-        async with self._send_lock:
+        async with self._send_conflict_detector:
             if self.socket.fileno() == -1:
                 raise ClosedStreamError
             with _translate_socket_errors_to_stream_errors():
                 await self.socket.wait_writable()
 
     async def send_eof(self):
-        async with self._send_lock:
+        async with self._send_conflict_detector:
             # On MacOS, calling shutdown a second time raises ENOTCONN, but
             # send_eof needs to be idempotent.
             if self.socket.did_shutdown_SHUT_WR:
diff --git a/trio/_ssl.py b/trio/_ssl.py
index 2564b30b1e..24f36817bb 100644
--- a/trio/_ssl.py
+++ b/trio/_ssl.py
@@ -159,7 +159,7 @@
     BrokenStreamError, ClosedStreamError, aclose_forcefully
 )
 from . import _sync
-from ._util import UnLock
+from ._util import ConflictDetector
 
 __all__ = ["SSLStream", "SSLListener"]
 
@@ -368,12 +368,10 @@ def __init__(
         # These are used to make sure that our caller doesn't attempt to make
         # multiple concurrent calls to send_all/wait_send_all_might_not_block
         # or to receive_some.
-        self._outer_send_lock = UnLock(
-            _core.ResourceBusyError,
+        self._outer_send_conflict_detector = ConflictDetector(
             "another task is currently sending data on this SSLStream"
         )
-        self._outer_recv_lock = UnLock(
-            _core.ResourceBusyError,
+        self._outer_recv_conflict_detector = ConflictDetector(
             "another task is currently receiving data on this SSLStream"
         )
 
@@ -624,7 +622,7 @@ async def receive_some(self, max_bytes):
            :exc:`trio.BrokenStreamError`.
 
         """
-        async with self._outer_recv_lock:
+        async with self._outer_recv_conflict_detector:
             self._check_status()
             try:
                 await self._handshook.ensure(checkpoint=False)
@@ -666,7 +664,7 @@ async def send_all(self, data):
            :exc:`trio.BrokenStreamError`.
 
         """
-        async with self._outer_send_lock:
+        async with self._outer_send_conflict_detector:
             self._check_status()
             await self._handshook.ensure(checkpoint=False)
             # SSLObject interprets write(b"") as an EOF for some reason, which
@@ -693,7 +691,8 @@ async def unwrap(self):
           ``transport_stream.receive_some(...)``.
 
         """
-        async with self._outer_recv_lock, self._outer_send_lock:
+        async with self._outer_recv_conflict_detector, \
+                   self._outer_send_conflict_detector:
             self._check_status()
             await self._handshook.ensure(checkpoint=False)
             await self._retry(self._ssl_object.unwrap)
@@ -797,7 +796,7 @@ async def wait_send_all_might_not_block(self):
         # semantics that wait_send_all_might_not_block and send_all
         # conflict. This also takes care of providing correct checkpoint
         # semantics before we potentially error out from _check_status().
-        async with self._outer_send_lock:
+        async with self._outer_send_conflict_detector:
             self._check_status()
             # Then we take the inner send lock. We know that no other tasks
             # are calling self.send_all or self.wait_send_all_might_not_block,
diff --git a/trio/_util.py b/trio/_util.py
index 32ee70d58b..8a39aa7765 100644
--- a/trio/_util.py
+++ b/trio/_util.py
@@ -8,16 +8,16 @@
 
 # There's a dependency loop here... _core is allowed to use this file (in fact
 # it's the *only* file in the main trio/ package it's allowed to use), but
-# UnLock needs yield_briefly so it also has to import _core. Possibly we
-# should split this file into two: one for true generic low-level utility
-# code, and one for higher level helpers?
+# ConflictDetector needs yield_briefly so it also has to import
+# _core. Possibly we should split this file into two: one for true generic
+# low-level utility code, and one for higher level helpers?
 from . import _core
 
 __all__ = [
     "signal_raise",
     "aiter_compat",
     "acontextmanager",
-    "UnLock",
+    "ConflictDetector",
     "fixup_module_metadata",
 ]
 
@@ -176,15 +176,14 @@ def helper(*args, **kwds):
     return helper
 
 
-class _UnLockSync:
-    def __init__(self, exc, *args):
-        self._exc = exc
-        self._args = args
+class _ConflictDetectorSync:
+    def __init__(self, msg):
+        self._msg = msg
         self._held = False
 
     def __enter__(self):
         if self._held:
-            raise self._exc(*self._args)
+            raise _core.ResourceBusyError(self._msg)
         else:
             self._held = True
 
@@ -192,8 +191,9 @@ def __exit__(self, *args):
         self._held = False
 
 
-class UnLock:
-    """An unnecessary lock.
+class ConflictDetector:
+    """Detect when two tasks are about to perform operations that would
+    conflict.
 
     Use as an async context manager; if two tasks enter it at the same
     time then the second one raises an error. You can use it when there are
@@ -205,10 +205,13 @@ class UnLock:
 
     This executes a checkpoint on entry. That's the only reason it's async.
 
+    To use from sync code, do ``with cd.sync``; this is just like ``async with
+    cd`` except that it doesn't execute a checkpoint.
+
     """
 
-    def __init__(self, exc, *args):
-        self.sync = _UnLockSync(exc, *args)
+    def __init__(self, msg):
+        self.sync = _ConflictDetectorSync(msg)
 
     async def __aenter__(self):
         await _core.yield_briefly()
diff --git a/trio/testing/_memory_streams.py b/trio/testing/_memory_streams.py
index e36dcec411..8b7e31fe18 100644
--- a/trio/testing/_memory_streams.py
+++ b/trio/testing/_memory_streams.py
@@ -27,8 +27,8 @@ def __init__(self):
         self._data = bytearray()
         self._closed = False
         self._lot = _core.ParkingLot()
-        self._fetch_lock = _util.UnLock(
-            _core.ResourceBusyError, "another task is already fetching data"
+        self._fetch_lock = _util.ConflictDetector(
+            "another task is already fetching data"
         )
 
     def close(self):
@@ -102,8 +102,8 @@ def __init__(
             wait_send_all_might_not_block_hook=None,
             close_hook=None
     ):
-        self._lock = _util.UnLock(
-            _core.ResourceBusyError, "another task is using this stream"
+        self._conflict_detector = _util.ConflictDetector(
+            "another task is using this stream"
         )
         self._outgoing = _UnboundedByteQueue()
         self.send_all_hook = send_all_hook
@@ -118,7 +118,7 @@ async def send_all(self, data):
         # The lock itself is a checkpoint, but then we also yield inside the
         # lock to give ourselves a chance to detect buggy user code that calls
         # this twice at the same time.
-        async with self._lock:
+        async with self._conflict_detector:
             await _core.yield_briefly()
             self._outgoing.put(data)
             if self.send_all_hook is not None:
@@ -132,7 +132,7 @@ async def wait_send_all_might_not_block(self):
         # The lock itself is a checkpoint, but then we also yield inside the
         # lock to give ourselves a chance to detect buggy user code that calls
         # this twice at the same time.
-        async with self._lock:
+        async with self._conflict_detector:
             await _core.yield_briefly()
             # check for being closed:
             self._outgoing.put(b"")
@@ -201,8 +201,8 @@ class MemoryReceiveStream(ReceiveStream):
     """
 
     def __init__(self, receive_some_hook=None, close_hook=None):
-        self._lock = _util.UnLock(
-            _core.ResourceBusyError, "another task is using this stream"
+        self._conflict_detector = _util.ConflictDetector(
+            "another task is using this stream"
         )
         self._incoming = _UnboundedByteQueue()
         self._closed = False
@@ -217,7 +217,7 @@ async def receive_some(self, max_bytes):
         # The lock itself is a checkpoint, but then we also yield inside the
         # lock to give ourselves a chance to detect buggy user code that calls
         # this twice at the same time.
-        async with self._lock:
+        async with self._conflict_detector:
             await _core.yield_briefly()
             if max_bytes is None:
                 raise TypeError("max_bytes must not be None")
@@ -435,11 +435,11 @@ def __init__(self):
         self._receiver_closed = False
         self._receiver_waiting = False
         self._waiters = _core.ParkingLot()
-        self._send_lock = _util.UnLock(
-            _core.ResourceBusyError, "another task is already sending"
+        self._send_conflict_detector = _util.ConflictDetector(
+            "another task is already sending"
         )
-        self._receive_lock = _util.UnLock(
-            _core.ResourceBusyError, "another task is already receiving"
+        self._receive_conflict_detector = _util.ConflictDetector(
+            "another task is already receiving"
         )
 
     def _something_happened(self):
@@ -459,7 +459,7 @@ def close_receiver(self):
         self._something_happened()
 
     async def send_all(self, data):
-        async with self._send_lock:
+        async with self._send_conflict_detector:
             if self._sender_closed:
                 raise ClosedStreamError
             if self._receiver_closed:
@@ -476,7 +476,7 @@ async def send_all(self, data):
                 return
 
     async def wait_send_all_might_not_block(self):
-        async with self._send_lock:
+        async with self._send_conflict_detector:
             if self._sender_closed:
                 raise ClosedStreamError
             if self._receiver_closed:
@@ -486,7 +486,7 @@ async def wait_send_all_might_not_block(self):
             )
 
     async def receive_some(self, max_bytes):
-        async with self._receive_lock:
+        async with self._receive_conflict_detector:
             # Argument validation
             max_bytes = operator.index(max_bytes)
             if max_bytes < 1:
diff --git a/trio/tests/test_ssl.py b/trio/tests/test_ssl.py
index 629c59163d..84509d21f1 100644
--- a/trio/tests/test_ssl.py
+++ b/trio/tests/test_ssl.py
@@ -20,7 +20,7 @@
 from .._highlevel_open_tcp_stream import open_tcp_stream
 from .. import ssl as tssl
 from .. import socket as tsocket
-from .._util import UnLock, acontextmanager
+from .._util import ConflictDetector, acontextmanager
 
 from .._core.tests.tutil import slow
 
@@ -175,12 +175,10 @@ def __init__(self, sleeper=None):
         self._lot = _core.ParkingLot()
         self._pending_cleartext = bytearray()
 
-        self._send_all_mutex = UnLock(
-            _core.ResourceBusyError,
+        self._send_all_conflict_detector = ConflictDetector(
             "simultaneous calls to PyOpenSSLEchoStream.send_all"
         )
-        self._receive_some_mutex = UnLock(
-            _core.ResourceBusyError,
+        self._receive_some_conflict_detector = ConflictDetector(
             "simultaneous calls to PyOpenSSLEchoStream.receive_some"
         )
 
@@ -205,13 +203,13 @@ def renegotiate(self):
         assert self._conn.renegotiate()
 
     async def wait_send_all_might_not_block(self):
-        async with self._send_all_mutex:
+        async with self._send_all_conflict_detector:
             await _core.yield_briefly()
             await self.sleeper("wait_send_all_might_not_block")
 
     async def send_all(self, data):
         print("  --> transport_stream.send_all")
-        async with self._send_all_mutex:
+        async with self._send_all_conflict_detector:
             await _core.yield_briefly()
             await self.sleeper("send_all")
             self._conn.bio_write(data)
@@ -233,7 +231,7 @@ async def send_all(self, data):
 
     async def receive_some(self, nbytes):
         print("  --> transport_stream.receive_some")
-        async with self._receive_some_mutex:
+        async with self._receive_some_conflict_detector:
             try:
                 await _core.yield_briefly()
                 while True:
diff --git a/trio/tests/test_util.py b/trio/tests/test_util.py
index 7a29012dd7..4765ff969f 100644
--- a/trio/tests/test_util.py
+++ b/trio/tests/test_util.py
@@ -25,40 +25,34 @@ def handler(signum, _):
     assert record == [signal.SIGFPE]
 
 
-async def test_UnLock():
-    ul1 = UnLock(RuntimeError, "ul1")
-    ul2 = UnLock(ValueError)
+async def test_ConflictDetector():
+    ul1 = ConflictDetector("ul1")
+    ul2 = ConflictDetector("ul2")
 
     async with ul1:
         with assert_yields():
             async with ul2:
                 print("ok")
 
-    with pytest.raises(RuntimeError) as excinfo:
+    with pytest.raises(_core.ResourceBusyError) as excinfo:
         async with ul1:
             with assert_yields():
                 async with ul1:
                     pass  # pragma: no cover
     assert "ul1" in str(excinfo.value)
 
-    with pytest.raises(ValueError) as excinfo:
-        async with ul2:
-            with assert_yields():
-                async with ul2:
-                    pass  # pragma: no cover
-
     async def wait_with_ul1():
         async with ul1:
             await wait_all_tasks_blocked()
 
-    with pytest.raises(RuntimeError) as excinfo:
+    with pytest.raises(_core.ResourceBusyError) as excinfo:
         async with _core.open_nursery() as nursery:
             nursery.spawn(wait_with_ul1)
             nursery.spawn(wait_with_ul1)
     assert "ul1" in str(excinfo.value)
 
     # mixing sync and async entry
-    with pytest.raises(RuntimeError) as excinfo:
+    with pytest.raises(_core.ResourceBusyError) as excinfo:
         with ul1.sync:
             with assert_yields():
                 async with ul1: