Add a note about acquiring/releasing by the same task

README.rst

@@ -24,6 +24,9 @@ frequently searched is an ideal candidate for the use of a read-write lock.
 However, if updates become frequent then the data spends most of its time
 being exclusively locked and there is little, if any increase in concurrency.
 
+**Note:** a task that *acquires* the lock should be used for *releasing* it.
+Locking from one task and releasing from another one generates ``RuntimeError``.
+
 
 Implementation is almost direct port from this patch_.
 

aiorwlock/__init__.py

@@ -145,8 +145,8 @@ def _release(self, lock_type: int) -> None:
 
         try:
             self._owning.remove((me, lock_type))
-        except ValueError:
-            raise RuntimeError('Cannot release an un-acquired lock')
+        except ValueError as exc:
+            raise RuntimeError('Cannot release an un-acquired lock') from exc
         if lock_type == self._RL:
             self._r_state -= 1
         else: