diff --git a/docs/api/gevent.hub.rst b/docs/api/gevent.hub.rst
index 9797ede63..36ffc970a 100644
--- a/docs/api/gevent.hub.rst
+++ b/docs/api/gevent.hub.rst
@@ -30,6 +30,7 @@ All implementations of the loop provide a common minimum interface.
 
 .. autointerface:: gevent._interfaces.ILoop
 .. autointerface:: gevent._interfaces.IWatcher
+.. autointerface:: gevent._interfaces.ICallback
 
 Utilities
 =========
diff --git a/docs/changes/1739.misc b/docs/changes/1739.misc
index d4edc7c13..be957f368 100644
--- a/docs/changes/1739.misc
+++ b/docs/changes/1739.misc
@@ -1,4 +1,4 @@
-Make ``AsyncResult`` print a warning when it detects improper
+Make ``gevent.event.AsyncResult`` print a warning when it detects improper
 cross-thread usage instead of hanging.
 
 ``AsyncResult`` has *never* been safe to use from multiple threads.
@@ -18,4 +18,8 @@ are still possible, especially under PyPy 7.3.3.
 At the same time, ``AsyncResult`` is tuned to behave more like it did
 in older versions, meaning that the hang is once again much less
 likely. If you were getting lucky and using ``AsyncResult``
-successfully across threads, this may restore your luck.
+successfully across threads, this may restore your luck. In addition,
+cross-thread wakeups are faster. Note that the gevent hub now uses an
+extra file descriptor to implement this.
+
+Similar changes apply to ``gevent.event.Event`` (see :issue:`1735`).
diff --git a/setup.py b/setup.py
index fd9e0d508..62882c73d 100755
--- a/setup.py
+++ b/setup.py
@@ -406,6 +406,7 @@ def run_setup(ext_modules):
             'docs': [
                 'repoze.sphinx.autointerface',
                 'sphinxcontrib-programoutput',
+                'zope.schema',
             ],
             # To the extent possible, we should work to make sure
             # our tests run, at least a basic set, without any of
diff --git a/src/gevent/_abstract_linkable.py b/src/gevent/_abstract_linkable.py
index 14ba4c66d..c475a1209 100644
--- a/src/gevent/_abstract_linkable.py
+++ b/src/gevent/_abstract_linkable.py
@@ -47,7 +47,11 @@ def get_roots_and_hubs():
     return {
         x.parent: x
         for x in get_objects()
-        if isinstance(x, Hub)
+        # Make sure to only find hubs that have a loop
+        # and aren't destroyed. If we don't do that, we can
+        # get an old hub that no longer works leading to issues in
+        # combined test cases.
+        if isinstance(x, Hub) and x.loop is not None
     }
 
 
@@ -393,9 +397,9 @@ def _handle_unswitched_notifications(self, unswitched):
                         root_greenlets = get_roots_and_hubs()
                     hub = root_greenlets.get(glet)
 
-                if hub is not None:
-                    hub.loop.run_callback(link, self)
-            if hub is None:
+                if hub is not None and hub.loop is not None:
+                    hub.loop.run_callback_threadsafe(link, self)
+            if hub is None or hub.loop is None:
                 # We couldn't handle it
                 self.__print_unswitched_warning(link, printed_tb)
                 printed_tb = True
diff --git a/src/gevent/_ffi/callback.py b/src/gevent/_ffi/callback.py
index e3b2d1afc..1b0d4f1be 100644
--- a/src/gevent/_ffi/callback.py
+++ b/src/gevent/_ffi/callback.py
@@ -1,10 +1,16 @@
-from __future__ import absolute_import, print_function
+from __future__ import absolute_import
+from __future__ import print_function
+
+from zope.interface import implementer
+
+from gevent._interfaces import ICallback
 
 __all__ = [
     'callback',
 ]
 
 
+@implementer(ICallback)
 class callback(object):
 
     __slots__ = ('callback', 'args')
diff --git a/src/gevent/_ffi/loop.py b/src/gevent/_ffi/loop.py
index 44b858eb4..8dd3e8c06 100644
--- a/src/gevent/_ffi/loop.py
+++ b/src/gevent/_ffi/loop.py
@@ -392,6 +392,7 @@ class AbstractLoop(object):
     _default = None
 
     _keepaliveset = _DiscardedSet()
+    _threadsafe_async = None
 
     def __init__(self, ffi, lib, watchers, flags=None, default=None):
         self._ffi = ffi
@@ -405,7 +406,6 @@ def __init__(self, ffi, lib, watchers, flags=None, default=None):
         self._keepaliveset = set()
         self._init_loop_and_aux_watchers(flags, default)
 
-
     def _init_loop_and_aux_watchers(self, flags=None, default=None):
         self._ptr = self._init_loop(flags, default)
 
@@ -436,6 +436,8 @@ def _init_loop_and_aux_watchers(self, flags=None, default=None):
         self._timer0.data = self._handle_to_self
         self._init_callback_timer()
 
+        self._threadsafe_async = self.async_(ref=False)
+        self._threadsafe_async.start(lambda: None)
         # TODO: We may be able to do something nicer and use the existing python_callback
         # combined with onerror and the class check/timer/prepare to simplify things
         # and unify our handling
@@ -546,7 +548,9 @@ def _run_callbacks(self): # pylint:disable=too-many-branches
             self.starting_timer_may_update_loop_time = False
 
     def _stop_aux_watchers(self):
-        raise NotImplementedError()
+        if self._threadsafe_async is not None:
+            self._threadsafe_async.close()
+            self._threadsafe_async = None
 
     def destroy(self):
         ptr = self.ptr
@@ -739,9 +743,13 @@ def run_callback(self, func, *args):
         # _run_callbacks), this could happen almost immediately,
         # without the loop cycling.
         cb = callback(func, args)
-        self._callbacks.append(cb)
-        self._setup_for_run_callback()
+        self._callbacks.append(cb) # Relying on the GIL for this to be threadsafe
+        self._setup_for_run_callback() # XXX: This may not be threadsafe.
+        return cb
 
+    def run_callback_threadsafe(self, func, *args):
+        cb = self.run_callback(func, *args)
+        self._threadsafe_async.send()
         return cb
 
     def _format(self):
diff --git a/src/gevent/_gevent_c_abstract_linkable.pxd b/src/gevent/_gevent_c_abstract_linkable.pxd
index 0f1be96c9..d2ec604b6 100644
--- a/src/gevent/_gevent_c_abstract_linkable.pxd
+++ b/src/gevent/_gevent_c_abstract_linkable.pxd
@@ -69,6 +69,7 @@ cdef class AbstractLinkable(object):
    @cython.nonecheck(False)
    cpdef _notify_links(self, list arrived_while_waiting)
 
+   @cython.locals(hub=SwitchOutGreenletWithLoop)
    cdef _handle_unswitched_notifications(self, list unswitched)
    cdef __print_unswitched_warning(self, link, bint printed_tb)
 
diff --git a/src/gevent/_interfaces.py b/src/gevent/_interfaces.py
index 119cdf855..05a5051f7 100644
--- a/src/gevent/_interfaces.py
+++ b/src/gevent/_interfaces.py
@@ -19,12 +19,31 @@
 from zope.interface import Interface
 from zope.interface import Attribute
 
+try:
+    from zope import schema
+except ImportError: # pragma: no cover
+    class _Field(Attribute):
+        __allowed_kw__ = ('readonly', 'min',)
+        def __init__(self, description, required=False, **kwargs):
+            description = "%s (required? %s)" % (description, required)
+            for k in self.__allowed_kw__:
+                kwargs.pop(k, None)
+            if kwargs:
+                raise TypeError("Unexpected keyword arguments: %r" % (kwargs,))
+            Attribute.__init__(self, description)
+
+    class schema(object):
+        Bool = _Field
+        Float = _Field
+
+
 # pylint:disable=no-method-argument, unused-argument, no-self-argument
 # pylint:disable=inherit-non-class
 
 __all__ = [
     'ILoop',
     'IWatcher',
+    'ICallback',
 ]
 
 class ILoop(Interface):
@@ -46,13 +65,20 @@ class ILoop(Interface):
     this watcher is still started. *priority* is event loop specific.
     """
 
-    default = Attribute("Boolean indicating whether this is the default loop")
+    default = schema.Bool(
+        description=u"Boolean indicating whether this is the default loop",
+        required=True,
+        readonly=True,
+    )
 
-    approx_timer_resolution = Attribute(
-        "Floating point number of seconds giving (approximately) the minimum "
+    approx_timer_resolution = schema.Float(
+        description=u"Floating point number of seconds giving (approximately) the minimum "
         "resolution of a timer (and hence the minimun value the sleep can sleep for). "
         "On libuv, this is fixed by the library, but on libev it is just a guess "
-        "and the actual value is system dependent."
+        "and the actual value is system dependent.",
+        required=True,
+        min=0.0,
+        readonly=True,
     )
 
     def run(nowait=False, once=False):
@@ -160,7 +186,7 @@ def fork(ref=True, priority=None):
         """
         Create a watcher that fires when the process forks.
 
-        Availability: POSIX
+        Availability: Unix.
         """
 
     def async_(ref=True, priority=None):
@@ -182,6 +208,8 @@ def child(pid, trace=0, ref=True):
             Create a watcher that fires for events on the child with process ID *pid*.
 
             This is platform specific and not available on Windows.
+
+            Availability: Unix.
             """
 
     def stat(path, interval=0.0, ref=True, priority=None):
@@ -196,8 +224,29 @@ def run_callback(func, *args):
         """
         Run the *func* passing it *args* at the next opportune moment.
 
-        This is a way of handing control to the event loop and deferring
-        an action.
+        The next opportune moment may be the next iteration of the event loop,
+        the current iteration, or some other time in the future.
+
+        Returns a :class:`ICallback` object. See that documentation for
+        important caveats.
+
+        .. seealso:: :meth:`asyncio.loop.call_soon`
+           The :mod:`asyncio` equivalent.
+        """
+
+    def run_callback_threadsafe(func, *args):
+        """
+        Like :meth:`run_callback`, but for use from *outside* the
+        thread that is running this loop.
+
+        This not only schedules the *func* to run, it also causes the
+        loop to notice that the *func* has been scheduled (e.g., it causes
+        the loop to wake up).
+
+        .. versionadded:: NEXT
+
+        .. seealso:: :meth:`asyncio.loop.call_soon_threadsafe`
+           The :mod:`asyncio` equivalent.
         """
 
 class IWatcher(Interface):
@@ -242,3 +291,25 @@ def close():
         undefined. You should dispose of any references you have to it
         after calling this method.
         """
+
+class ICallback(Interface):
+    """
+    Represents a function that will be run some time in the future.
+
+    Callback functions run in the hub, and as such they cannot use
+    gevent's blocking API; any exception they raise cannot be caught.
+    """
+
+    pending = schema.Bool(description="Has this callback run yet?",
+                          readonly=True)
+
+    def stop():
+        """
+        If this object is still `pending`, cause it to
+        no longer be `pending`; the function will not be run.
+        """
+
+    def close():
+        """
+        An alias of `stop`.
+        """
diff --git a/src/gevent/_semaphore.py b/src/gevent/_semaphore.py
index 9f70a4d32..b86cea68b 100644
--- a/src/gevent/_semaphore.py
+++ b/src/gevent/_semaphore.py
@@ -405,7 +405,7 @@ def __acquire_using_other_hub(self, owning_hub, timeout):
         thread_lock.acquire()
         results = []
 
-        owning_hub.loop.run_callback(
+        owning_hub.loop.run_callback_threadsafe(
             spawn_raw,
             self.__acquire_from_other_thread_cb,
             results,
diff --git a/src/gevent/libev/corecext.pyx b/src/gevent/libev/corecext.pyx
index 919f3c328..7fbf723e9 100644
--- a/src/gevent/libev/corecext.pyx
+++ b/src/gevent/libev/corecext.pyx
@@ -392,6 +392,7 @@ cdef public class loop [object PyGeventLoopObject, type PyGeventLoop_Type]:
     ## embedded struct members
     cdef libev.ev_prepare _prepare
     cdef libev.ev_timer _timer0
+    cdef libev.ev_async _threadsafe_async
     # We'll only actually start this timer if we're on Windows,
     # but it doesn't hurt to compile it in on all platforms.
     cdef libev.ev_timer _periodic_signal_checker
@@ -421,6 +422,8 @@ cdef public class loop [object PyGeventLoopObject, type PyGeventLoop_Type]:
         libev.ev_timer_init(&self._timer0,
                             <void*>gevent_noop,
                             0.0, 0.0)
+        libev.ev_async_init(&self._threadsafe_async,
+                            <void*>gevent_noop)
 
         cdef unsigned int c_flags
         if ptr:
@@ -454,6 +457,10 @@ cdef public class loop [object PyGeventLoopObject, type PyGeventLoop_Type]:
         libev.ev_prepare_start(self._ptr, &self._prepare)
         libev.ev_unref(self._ptr)
 
+        libev.ev_async_start(self._ptr, &self._threadsafe_async)
+        libev.ev_unref(self._ptr)
+
+
     def __init__(self, object flags=None, object default=None, libev.intptr_t ptr=0):
         self._callbacks = CallbackFIFO()
         # See libev.corecffi for this attribute.
@@ -507,6 +514,9 @@ cdef public class loop [object PyGeventLoopObject, type PyGeventLoop_Type]:
         if libev.ev_is_active(&self._periodic_signal_checker):
             libev.ev_ref(ptr)
             libev.ev_timer_stop(ptr, &self._periodic_signal_checker)
+        if libev.ev_is_active(&self._threadsafe_async):
+            libev.ev_ref(ptr)
+            libev.ev_async_stop(ptr, &self._threadsafe_async)
 
     def destroy(self):
         cdef libev.ev_loop* ptr = self._ptr
@@ -520,8 +530,8 @@ cdef public class loop [object PyGeventLoopObject, type PyGeventLoop_Type]:
                 # else with it will likely cause a crash.
                 return
             # Mark as destroyed
-            libev.ev_set_userdata(ptr, NULL)
             self._stop_watchers(ptr)
+            libev.ev_set_userdata(ptr, NULL)
             if SYSERR_CALLBACK == self._handle_syserr:
                 set_syserr_cb(None)
             libev.ev_loop_destroy(ptr)
@@ -715,6 +725,12 @@ cdef public class loop [object PyGeventLoopObject, type PyGeventLoop_Type]:
         libev.ev_ref(self._ptr)
         return cb
 
+    def run_callback_threadsafe(self, func, *args):
+        # We rely on the GIL to make this threadsafe.
+        cb = self.run_callback(func, *args)
+        libev.ev_async_send(self._ptr, &self._threadsafe_async)
+        return cb
+
     def _format(self):
         if not self._ptr:
             return 'destroyed'
@@ -775,15 +791,17 @@ cdef public class loop [object PyGeventLoopObject, type PyGeventLoop_Type]:
         # Explicitly not EV_USE_SIGNALFD
         raise AttributeError("sigfd")
 
-try:
-    from zope.interface import classImplements
-except ImportError:
-    pass
-else:
-    # XXX: This invokes the side-table lookup, we would
-    # prefer to have it stored directly on the class.
-    from gevent._interfaces import ILoop
-    classImplements(loop, ILoop)
+
+from zope.interface import classImplements
+
+# XXX: This invokes the side-table lookup, we would
+# prefer to have it stored directly on the class. That means we
+# need a class variable ``__implemented__``, but that's hard in
+# Cython
+from gevent._interfaces import ILoop
+from gevent._interfaces import ICallback
+classImplements(loop, ILoop)
+classImplements(callback, ICallback)
 
 # about readonly _flags attribute:
 # bit #1 set if object owns Python reference to itself (Py_INCREF was
diff --git a/src/gevent/libev/corecffi.py b/src/gevent/libev/corecffi.py
index aa96cc821..d40e874de 100644
--- a/src/gevent/libev/corecffi.py
+++ b/src/gevent/libev/corecffi.py
@@ -284,6 +284,7 @@ def _start_callback_timer(self):
         libev.ev_timer_start(self._ptr, self._timer0)
 
     def _stop_aux_watchers(self):
+        super(loop, self)._stop_aux_watchers()
         if libev.ev_is_active(self._prepare):
             self.ref()
             libev.ev_prepare_stop(self._ptr, self._prepare)
diff --git a/src/gevent/libuv/loop.py b/src/gevent/libuv/loop.py
index 21fc96067..877e82db6 100644
--- a/src/gevent/libuv/loop.py
+++ b/src/gevent/libuv/loop.py
@@ -334,6 +334,7 @@ def _start_callback_timer(self):
 
 
     def _stop_aux_watchers(self):
+        super(loop, self)._stop_aux_watchers()
         assert self._prepare
         assert self._check
         assert self._signal_idle
@@ -456,7 +457,8 @@ def unref(self):
         pass
 
     def break_(self, how=None):
-        libuv.uv_stop(self.ptr)
+        if self.ptr:
+            libuv.uv_stop(self.ptr)
 
     def reinit(self):
         # TODO: How to implement? We probably have to simply
diff --git a/src/gevent/tests/test__event.py b/src/gevent/tests/test__event.py
index bf0dc7896..824749f69 100644
--- a/src/gevent/tests/test__event.py
+++ b/src/gevent/tests/test__event.py
@@ -140,6 +140,26 @@ def test_nonblocking_get(self):
         self.assertRaises(gevent.Timeout, ar.get, block=False)
         self.assertRaises(gevent.Timeout, ar.get_nowait)
 
+class TestAsyncResultCrossThread(greentest.TestCase):
+
+    def _makeOne(self):
+        return AsyncResult()
+
+    def _setOne(self, one):
+        one.set('from main')
+
+    BG_WAIT_DELAY = 60
+
+    def _check_pypy_switch(self):
+        # On PyPy 7.3.3, switching to the main greenlet of a thread from a
+        # different thread silently does nothing. We can't detect the cross-thread
+        # switch, and so this test breaks
+        # https://foss.heptapod.net/pypy/pypy/-/issues/3381
+        if greentest.PYPY:
+            import sys
+            if sys.pypy_version_info[:3] <= (7, 3, 3): # pylint:disable=no-member
+                self.skipTest("PyPy bug: https://foss.heptapod.net/pypy/pypy/-/issues/3381")
+
     @greentest.ignores_leakcheck
     def test_cross_thread_use(self, timed_wait=False, wait_in_bg=False):
         # Issue 1739.
@@ -153,15 +173,10 @@ def test_cross_thread_use(self, timed_wait=False, wait_in_bg=False):
         from threading import Thread as NativeThread
         from threading import Event as NativeEvent
 
-        # On PyPy 7.3.3, switching to the main greenlet of a thread from a
-        # different thread silently does nothing. We can't detect the cross-thread
-        # switch, and so this test breaks
-        # https://foss.heptapod.net/pypy/pypy/-/issues/3381
-        if not wait_in_bg and greentest.PYPY:
-            import sys
-            if sys.pypy_version_info[:3] <= (7, 3, 3): # pylint:disable=no-member
-                self.skipTest("PyPy bug: https://foss.heptapod.net/pypy/pypy/-/issues/3381")
+        if not wait_in_bg:
+            self._check_pypy_switch()
 
+        test = self
         class Thread(NativeThread):
             def __init__(self):
                 NativeThread.__init__(self)
@@ -169,7 +184,7 @@ def __init__(self):
                 self.running_event = NativeEvent()
                 self.finished_event = NativeEvent()
 
-                self.async_result = AsyncResult()
+                self.async_result = test._makeOne()
                 self.result = '<never set>'
 
             def run(self):
@@ -182,10 +197,12 @@ def spin():
 
                 def work():
                     self.running_event.set()
-                    # XXX: If we use a timed wait(), the bug doesn't manifest.
-                    # Why not?
+                    # If we use a timed wait(), the bug doesn't manifest.
+                    # This is probably because the loop wakes up to handle the timer,
+                    # and notices the callback.
+                    # See https://github.com/gevent/gevent/issues/1735
                     if timed_wait:
-                        self.result = self.async_result.wait(DELAY * 5)
+                        self.result = self.async_result.wait(test.BG_WAIT_DELAY)
                     else:
                         self.result = self.async_result.wait()
 
@@ -207,12 +224,15 @@ def work():
         thread.start()
         try:
             thread.running_event.wait()
-            thread.async_result.set('from main')
+            self._setOne(thread.async_result)
             thread.finished_event.wait(DELAY * 5)
         finally:
             thread.join(DELAY * 15)
 
-        self.assertEqual(thread.result, 'from main')
+        self._check_result(thread.result)
+
+    def _check_result(self, result):
+        self.assertEqual(result, 'from main')
 
     def test_cross_thread_use_bg(self):
         self.test_cross_thread_use(timed_wait=False, wait_in_bg=True)
@@ -223,6 +243,57 @@ def test_cross_thread_use_timed(self):
     def test_cross_thread_use_timed_bg(self):
         self.test_cross_thread_use(timed_wait=True, wait_in_bg=True)
 
+    @greentest.ignores_leakcheck
+    def test_cross_thread_use_set_in_bg(self):
+        self.assertNotMonkeyPatched() # Need real threads, event objects
+        from threading import Thread as NativeThread
+        from threading import Event as NativeEvent
+
+        self._check_pypy_switch()
+        test = self
+        class Thread(NativeThread):
+            def __init__(self):
+                NativeThread.__init__(self)
+                self.daemon = True
+                self.running_event = NativeEvent()
+                self.finished_event = NativeEvent()
+
+                self.async_result = test._makeOne()
+                self.result = '<never set>'
+
+            def run(self):
+                self.running_event.set()
+                test._setOne(self.async_result)
+
+                self.finished_event.set()
+                gevent.get_hub().destroy(destroy_loop=True)
+
+        thread = Thread()
+        try:
+            glet = gevent.spawn(thread.start)
+            result = thread.async_result.wait(self.BG_WAIT_DELAY)
+        finally:
+            thread.join(DELAY * 15)
+            glet.join(DELAY)
+        self._check_result(result)
+
+    @greentest.ignores_leakcheck
+    def test_cross_thread_use_set_in_bg2(self):
+        # Do it again to make sure it works multiple times.
+        self.test_cross_thread_use_set_in_bg()
+
+class TestEventCrossThread(TestAsyncResultCrossThread):
+
+    def _makeOne(self):
+        return Event()
+
+    def _setOne(self, one):
+        one.set()
+
+    def _check_result(self, result):
+        self.assertTrue(result)
+
+
 class TestAsyncResultAsLinkTarget(greentest.TestCase):
     error_fatal = False
 
diff --git a/src/gevent/tests/test__hub.py b/src/gevent/tests/test__hub.py
index 26d192237..5ffa0cea2 100644
--- a/src/gevent/tests/test__hub.py
+++ b/src/gevent/tests/test__hub.py
@@ -337,6 +337,24 @@ def test_implemensts_ILoop(self):
 
         verify.verifyObject(ILoop, loop)
 
+    def test_callback_implements_ICallback(self):
+        from gevent.testing import verify
+        from gevent._interfaces import ICallback
+
+        loop = get_hub().loop
+
+        cb = loop.run_callback(lambda: None)
+        verify.verifyObject(ICallback, cb)
+
+    def test_callback_ts_implements_ICallback(self):
+        from gevent.testing import verify
+        from gevent._interfaces import ICallback
+
+        loop = get_hub().loop
+
+        cb = loop.run_callback_threadsafe(lambda: None)
+        verify.verifyObject(ICallback, cb)
+
 
 class TestHandleError(unittest.TestCase):