Index: kern_umtx.c
===================================================================
--- kern_umtx.c	(revision 197734)
+++ kern_umtx.c	(working copy)
@@ -31,6 +31,7 @@
 #include "opt_compat.h"
 #include <sys/param.h>
 #include <sys/kernel.h>
+#include <sys/ktr.h>
 #include <sys/limits.h>
 #include <sys/lock.h>
 #include <sys/malloc.h>
@@ -2482,7 +2483,11 @@
 sleep:
 		/* contention bit is set, before sleeping, increase read waiter count */
 		blocked_readers = fuword32(&rwlock->rw_blocked_readers);
-		suword32(&rwlock->rw_blocked_readers, blocked_readers+1);
+		if (suword32(&rwlock->rw_blocked_readers,
+		    blocked_readers+1) == -1)
+			panic("Unhandled");
+		CTR4(KTR_SPARE2, "td = %p, lock = %p, bumped to: %d, state: %d",
+		    curthread, rwlock, blocked_readers+1, rwlock->rw_state);
 
 		while (state & wrflags) {
 			umtxq_lock(&uq->uq_key);
@@ -2501,7 +2506,11 @@
 
 		/* decrease read waiter count, and may clear read contention bit */
 		blocked_readers = fuword32(&rwlock->rw_blocked_readers);
-		suword32(&rwlock->rw_blocked_readers, blocked_readers-1);
+		if (suword32(&rwlock->rw_blocked_readers,
+		    blocked_readers-1) == -1)
+			panic("Unhandled");
+		CTR4(KTR_SPARE2, "td = %p, lock = %p, dec to: %d, state: %d",
+		    curthread, rwlock, blocked_readers-1, rwlock->rw_state);
 		if (blocked_readers == 1) {
 			state = fuword32(__DEVOLATILE(int32_t *, &rwlock->rw_state));
 			for (;;) {