Index: kern_umtx.c
===================================================================
--- kern_umtx.c	(revision 197734)
+++ kern_umtx.c	(working copy)
@@ -2444,11 +2444,17 @@
 		/* try to lock it */
 		while (!(state & wrflags)) {
 			if (__predict_false(URWLOCK_READER_COUNT(state) == URWLOCK_MAX_READERS)) {
+				CTR2(KTR_SPARE2,
+			    "td = %p, lock = %p, MaxReaders reached",
+				    curthread, rwlock);
 				umtx_key_release(&uq->uq_key);
 				return (EAGAIN);
 			}
 			oldstate = casuword32(&rwlock->rw_state, state, state + 1);
 			if (oldstate == state) {
+				CTR4(KTR_SPARE2,
+			    "td = %p, lock = %p, oldstate: 0x%x state: 0x%x",
+				    curthread, rwlock, state, state + 1);
 				umtx_key_release(&uq->uq_key);
 				return (0);
 			}
@@ -2466,13 +2472,21 @@
 		/* set read contention bit */
 		while ((state & wrflags) && !(state & URWLOCK_READ_WAITERS)) {
 			oldstate = casuword32(&rwlock->rw_state, state, state | URWLOCK_READ_WAITERS);
-			if (oldstate == state)
+			if (oldstate == state) {
+				CTR4(KTR_SPARE2,
+			    "td = %p, lock = %p, oldstate: 0x%x state: 0x%x",
+				    curthread, rwlock, state,
+				    state | URWLOCK_READ_WAITERS);
 				goto sleep;
+			}
 			state = oldstate;
 		}
 
 		/* state is changed while setting flags, restart */
 		if (!(state & wrflags)) {
+				CTR2(KTR_SPARE2,
+			    "td = %p, lock = %p State Changed While Setting FR",
+				    curthread, rwlock);
 			umtxq_lock(&uq->uq_key);
 			umtxq_unbusy(&uq->uq_key);
 			umtxq_unlock(&uq->uq_key);
@@ -2507,8 +2521,13 @@
 			for (;;) {
 				oldstate = casuword32(&rwlock->rw_state, state,
 					 state & ~URWLOCK_READ_WAITERS);
-				if (oldstate == state)
+				if (oldstate == state) {
+					CTR4(KTR_SPARE2,
+			    "td = %p, lock = %p, oldstate: 0x%x state: 0x%x",
+					    curthread, rwlock, state,
+					    state & ~URWLOCK_READ_WAITERS);
 					break;
+				}
 				state = oldstate;
 			}
 		}
@@ -2569,6 +2588,10 @@
 		while (!(state & URWLOCK_WRITE_OWNER) && URWLOCK_READER_COUNT(state) == 0) {
 			oldstate = casuword32(&rwlock->rw_state, state, state | URWLOCK_WRITE_OWNER);
 			if (oldstate == state) {
+				CTR4(KTR_SPARE2,
+			    "td = %p, lock = %p, oldstate: 0x%x state: 0x%x",
+				    curthread, rwlock, state,
+				    state | URWLOCK_WRITE_OWNER);
 				umtx_key_release(&uq->uq_key);
 				return (0);
 			}
@@ -2586,12 +2609,20 @@
 		while (((state & URWLOCK_WRITE_OWNER) || URWLOCK_READER_COUNT(state) != 0) &&
 		       (state & URWLOCK_WRITE_WAITERS) == 0) {
 			oldstate = casuword32(&rwlock->rw_state, state, state | URWLOCK_WRITE_WAITERS);
-			if (oldstate == state)
+			if (oldstate == state) {
+				CTR4(KTR_SPARE2,
+			    "td = %p, lock = %p, oldstate: 0x%x state: 0x%x",
+				    curthread, rwlock, state,
+				    state | URWLOCK_WRITE_WAITERS);
 				goto sleep;
+			}
 			state = oldstate;
 		}
 
 		if (!(state & URWLOCK_WRITE_OWNER) && URWLOCK_READER_COUNT(state) == 0) {
+			CTR2(KTR_SPARE2,
+			    "td = %p, lock = %p State Changed While Setting FW",
+			    curthread, rwlock);
 			umtxq_lock(&uq->uq_key);
 			umtxq_unbusy(&uq->uq_key);
 			umtxq_unlock(&uq->uq_key);
@@ -2623,8 +2654,13 @@
 			for (;;) {
 				oldstate = casuword32(&rwlock->rw_state, state,
 					 state & ~URWLOCK_WRITE_WAITERS);
-				if (oldstate == state)
+				if (oldstate == state) {
+					CTR4(KTR_SPARE2,
+			    "td = %p, lock = %p, oldstate: 0x%x state: 0x%x",
+					    curthread, rwlock, state,
+					    state & ~URWLOCK_WRITE_WAITERS);
 					break;
+				}
 				state = oldstate;
 			}
 		}
@@ -2691,8 +2727,13 @@
 					error = EPERM;
 					goto out;
 				}
-			} else
+			} else {
+				CTR4(KTR_SPARE2,
+			    "td = %p, lock = %p, oldstate: 0x%x state: 0x%x",
+				    curthread, rwlock, state,
+				    state & ~URWLOCK_WRITE_OWNER);
 				break;
+			}
 		}
 	} else if (URWLOCK_READER_COUNT(state) != 0) {
 		for (;;) {
@@ -2705,8 +2746,12 @@
 					goto out;
 				}
 			}
-			else
+			else {
+				CTR4(KTR_SPARE2,
+			    "td = %p, lock = %p, oldstate: 0x%x state: 0x%x",
+				    curthread, rwlock, state, state - 1);
 				break;
+			}
 		}
 	} else {
 		error = EPERM;