--- //depot/projects/smpng/sys/kern/subr_turnstile.c	2003/11/19 08:35:27
+++ //depot/user/jhb/lock/kern/subr_turnstile.c	2003/12/03 11:42:37
@@ -254,6 +259,25 @@
 		}
 
 		td1 = TAILQ_PREV(td, threadqueue, td_lockq);
+		if (td1 == NULL) {
+			printf("%s: td1 is NULL dang it\n", __func__);
+			printf(
+		    "td %p, ts %p ; td->td_blocked %p, ts->ts_lockobj %p\n",
+			    td, ts, td->td_blocked, ts->ts_lockobj);
+			printf("blocked threads:\n");
+			TAILQ_FOREACH(td1, &ts->ts_blocked, td_lockq) {
+				printf("\t%p (pid %d) %s\n", td1,
+				    td1->td_proc->p_pid, td1->td_proc->p_comm);
+			}
+			printf("pending threads:\n");
+			TAILQ_FOREACH(td1, &ts->ts_blocked, td_lockq) {
+				printf("\t%p (pid %d) %s\n", td1,
+				    td1->td_proc->p_pid, td1->td_proc->p_comm);
+			}
+			mtx_unlock_spin(&tc->tc_lock);
+			Debugger("yowzers batman");
+			return;
+		}
 		if (td1->td_priority <= pri) {
 			mtx_unlock_spin(&tc->tc_lock);
 			continue;