diff --git a/Lib/multiprocessing/util.py b/Lib/multiprocessing/util.py
index 394cc44..2920f24 100644
--- a/Lib/multiprocessing/util.py
+++ b/Lib/multiprocessing/util.py
@@ -265,6 +265,9 @@
     else:
         f = lambda p : p[0][0] is not None and p[0][0] >= minpriority
 
+    # Careful: _finalizer_registry may be mutated while this function
+    # is running (either by a GC run or by another thread).
+
     items = [x for x in _finalizer_registry.items() if f(x)]
     items.sort(reverse=True)
 
diff --git a/Lib/test/test_multiprocessing.py b/Lib/test/test_multiprocessing.py
index 8a6bb78..9fcd3bd 100644
--- a/Lib/test/test_multiprocessing.py
+++ b/Lib/test/test_multiprocessing.py
@@ -2088,6 +2088,14 @@
 
     ALLOWED_TYPES = ('processes',)
 
+    def setUp(self):
+        self.registry_backup = util._finalizer_registry.copy()
+        util._finalizer_registry.clear()
+
+    def tearDown(self):
+        self.assertFalse(util._finalizer_registry)
+        util._finalizer_registry.update(self.registry_backup)
+
     @classmethod
     def _test_finalize(cls, conn):
         class Foo(object):
@@ -2137,6 +2145,59 @@
         result = [obj for obj in iter(conn.recv, 'STOP')]
         self.assertEqual(result, ['a', 'b', 'd10', 'd03', 'd02', 'd01', 'e'])
 
+    def test_thread_safety(self):
+        # bpo-24484: _run_finalizers() should be thread-safe
+        def cb():
+            pass
+
+        class Foo(object):
+            def __init__(self):
+                self.ref = self  # create reference cycle
+                # insert finalizer at random key
+                util.Finalize(self, cb, exitpriority=random.randint(1, 100))
+
+        finish = False
+        exc = []
+
+        def run_finalizers():
+            while not finish:
+                time.sleep(random.random() * 1e-1)
+                try:
+                    # A GC run will eventually happen during this,
+                    # collecting stale Foo's and mutating the registry
+                    util._run_finalizers()
+                except Exception as e:
+                    exc.append(e)
+
+        def make_finalizers():
+            d = {}
+            while not finish:
+                try:
+                    # Old Foo's get gradually replaced and later
+                    # collected by the GC (because of the cyclic ref)
+                    d[random.getrandbits(5)] = {Foo() for i in range(10)}
+                except Exception as e:
+                    exc.append(e)
+                    d.clear()
+
+        old_interval = sys.getcheckinterval()
+        old_threshold = gc.get_threshold()
+        try:
+            sys.setcheckinterval(10)
+            gc.set_threshold(5, 5, 5)
+            threads = [threading.Thread(target=run_finalizers),
+                       threading.Thread(target=make_finalizers)]
+            with test_support.start_threads(threads):
+                time.sleep(4.0)  # Wait a bit to trigger race condition
+                finish = True
+            if exc:
+                raise exc[0]
+        finally:
+            sys.setcheckinterval(old_interval)
+            gc.set_threshold(*old_threshold)
+            gc.collect()  # Collect remaining Foo's
+
+
 #
 # Test that from ... import * works for each module
 #