diff --git a/mcs/class/System.Core/System.Threading/ReaderWriterLockSlim.cs b/mcs/class/System.Core/System.Threading/ReaderWriterLockSlim.cs
index eb42055eda9fcfa54e24274b981e3a38e0b4ef8d..07f1dd1f9e7c5c3aa415f16e57737fb2ca2e6f43 100644
--- a/mcs/class/System.Core/System.Threading/ReaderWriterLockSlim.cs
+++ b/mcs/class/System.Core/System.Threading/ReaderWriterLockSlim.cs
@@ -292,7 +292,8 @@ namespace System.Threading {
 
 			ctstate.LockState ^= LockState.Upgradable;
 			ctstate.UpgradeableRecursiveCount--;
-			Interlocked.Add (ref rwlock, -RwRead);
+			if (Interlocked.Add (ref rwlock, -RwRead) >> RwReadBit == 0)
+				readerDoneEvent.Set ();
 		}
 
 		public void Dispose ()