diff --git a/requests_mock/mocker.py b/requests_mock/mocker.py
index 3b787eb..095230d 100644
--- a/requests_mock/mocker.py
+++ b/requests_mock/mocker.py
@@ -10,7 +10,9 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+import contextlib
 import functools
+import sys
 import threading
 import types
 
@@ -30,7 +32,26 @@
 
 _original_send = requests.Session.send
 
-_send_lock = threading.Lock()
+# NOTE(phodge): we need to use an RLock (reentrant lock) here because
+# requests.Session.send() is reentrant. See further comments where we
+# monkeypatch get_adapter()
+_send_lock = threading.RLock()
+
+
+@contextlib.contextmanager
+def threading_rlock(timeout):
+    kwargs = {}
+    if sys.version_info.major >= 3:
+        # python2 doesn't support the timeout argument
+        kwargs['timeout'] = timeout
+
+    if not _send_lock.acquire(**kwargs):
+        raise Exception("Could not acquire threading lock - possible deadlock scenario")
+
+    try:
+        yield
+    finally:
+        _send_lock.release()
 
 
 def _is_bound_method(method):
@@ -134,8 +155,17 @@ def _fake_send(session, request, **kwargs):
             # are multiple threads running - one thread could restore the
             # original get_adapter() just as a second thread is about to
             # execute _original_send() below
-            with _send_lock:
+            with threading_rlock(timeout=10):
                 # mock get_adapter
+                #
+                # NOTE(phodge): requests.Session.send() is actually
+                # reentrant due to how it resolves redirects with nested
+                # calls to send(), however the reentry occurs _after_ the
+                # call to self.get_adapter(), so it doesn't matter that we
+                # will restore _last_get_adapter before a nested send() has
+                # completed as long as we monkeypatch get_adapter() each
+                # time immediately before calling original send() like we
+                # are doing here.
                 _set_method(session, "get_adapter", _fake_get_adapter)
 
                 # NOTE(jamielennox): self._last_send vs _original_send. Whilst it