--- xnu/xnu-517.9.4/iokit/Kernel/IOLocks.cpp
+++ xnu/xnu-201.5/iokit/Kernel/IOLocks.cpp
@@ -149,16 +149,21 @@
     assert(lock->thread == IOThreadSelf());
     assert(lock->count == 1 || interType == THREAD_UNINT);
     
+    assert_wait((event_t) event, (int) interType);
     lock->count = 0;
     lock->thread = 0;
-    res = thread_sleep_mutex((event_t) event, lock->mutex, (int) interType);
-
-    // Must re-establish the recursive lock no matter why we woke up
-    // otherwise we would potentially leave the return path corrupted.
-    assert(lock->thread == 0);
-    assert(lock->count == 0);
-    lock->thread = IOThreadSelf();
-    lock->count = count;
+    mutex_unlock(lock->mutex);
+    
+    res = thread_block(0);
+
+    if (THREAD_AWAKENED == res) {
+        mutex_lock(lock->mutex);
+        assert(lock->thread == 0);
+        assert(lock->count == 0);
+        lock->thread = IOThreadSelf();
+        lock->count = count;
+    }
+
     return res;
 }