Index: sys/kern/sys_process.c
===================================================================
--- sys/kern/sys_process.c
+++ sys/kern/sys_process.c
@@ -1129,24 +1129,35 @@
 		}
 
 	sendsig:
-		/*
-		 * Clear the pending event for the thread that just
-		 * reported its event (p_xthread).  This may not be
-		 * the thread passed to PT_CONTINUE, PT_STEP, etc. if
-		 * the debugger is resuming a different thread.
-		 */
-		td2 = p->p_xthread;
+		/* proctree_locked is true for all but PT_KILL. */
 		if (proctree_locked) {
 			sx_xunlock(&proctree_lock);
 			proctree_locked = 0;
 		}
+		
 		p->p_xsig = data;
-		p->p_xthread = NULL;
 		if ((p->p_flag & (P_STOPPED_SIG | P_STOPPED_TRACE)) != 0) {
-			/* deliver or queue signal */
-			td2->td_dbgflags &= ~TDB_XSIG;
-			td2->td_xsig = data;
+			if ((p->p_flag & P_STOPPED_TRACE) != 0) {
+				/* 
+				 * Clear the pending event for the
+				 * thread that just reported its event
+				 * (p_xthread).  This may not be the
+				 * thread passed to PT_CONTINUE,
+				 * PT_STEP, etc. if the debugger is
+				 * resuming a different thread.
+				 *
+				 * Deliver the pending signal via the
+				 * reporting thread in this case rather
+				 * than the process.
+				 */
+				MPASS(p->p_xthread != NULL);
+				p->p_xthread->td_dbgflags &= ~TDB_XSIG;
+				p->p_xthread->td_xsig = data;
+				p->p_xthread = NULL;
+			}
 
+			/* Can p_xsig assignment be moved here? */
+
 			/*
 			 * P_WKILLED is insurance that a PT_KILL/SIGKILL always
 			 * works immediately, even if another thread is
@@ -1161,21 +1172,41 @@
 				FOREACH_THREAD_IN_PROC(p, td3)
 					td3->td_dbgflags &= ~TDB_SUSPEND;
 			}
+
 			/*
-			 * unsuspend all threads, to not let a thread run,
-			 * you should use PT_SUSPEND to suspend it before
-			 * continuing process.
+			 * Unsuspend all threads.  To leave a thread
+			 * suspended, use PT_SUSPEND to suspend it
+			 * before continuing the process.
 			 */
 			PROC_SLOCK(p);
 			p->p_flag &= ~(P_STOPPED_TRACE|P_STOPPED_SIG|P_WAITED);
 			thread_unsuspend(p);
 			PROC_SUNLOCK(p);
+
+			/*
+			 * XXX: Should this instead be
+			 * if (orig_p_xthread == NULL) to
+			 * handle PT_KILL when p_xthread is not set but
+			 * P_STOPPED_SIG is set?  Can that even happen?
+			 * One can only PT_KILL a traced process, and a
+			 * traced process should not have P_STOPPED_SIGNAL
+			 * set and not also have P_STOPPED_TRACE, so perhaps
+			 * req == PT_ATTACH is sufficient?
+			 */
 			if (req == PT_ATTACH)
 				kern_psignal(p, data);
 		} else {
+			/*
+			 * XXX: Isn't this always true?  All but
+			 * PT_ATTACH require P_STOPPED_SIG or
+			 * P_STOPPED_TRACE, and PT_ATTACH hardcodes
+			 * data to SIGTOP.
+			 */
+			MPASS(data != 0);
 			if (data)
 				kern_psignal(p, data);
 		}
+		MPASS(p->p_xthread == NULL);
 		break;
 
 	case PT_WRITE_I: