Index: sys/kern/subr_syscall.c
===================================================================
--- sys/kern/subr_syscall.c
+++ sys/kern/subr_syscall.c
@@ -69,7 +69,9 @@
 	if (__predict_false(td->td_cowgen != p->p_cowgen))
 		thread_cow_update(td);
 	traced = (p->p_flag & P_TRACED) != 0;
-	if (__predict_false(traced || td->td_dbgflags & TDB_USERWR)) {
+	// if we're not traced, we won't care about TDB_USERWR; debugger cannot
+	// attach and set it before we get to callp->sy_call().
+	if (__predict_false(traced)) {
 		PROC_LOCK(p);
 		td->td_dbgflags &= ~TDB_USERWR;
 		if (traced)
@@ -90,25 +92,25 @@
 		goto retval;
 	}
 
-	if (__predict_false((p->p_flag & P_TRACED) != 0)) {
+	if (__predict_false(traced)) {
 		PROC_LOCK(p);
 		if (p->p_ptevents & PTRACE_SCE)
 			ptracestop((td), SIGTRAP, NULL);
 		PROC_UNLOCK(p);
-	}
-	if (__predict_false((td->td_dbgflags & TDB_USERWR) != 0)) {
-		/*
-		 * Reread syscall number and arguments if debugger
-		 * modified registers or memory.
-		 */
-		error = (p->p_sysent->sv_fetch_syscall_args)(td);
+		if (__predict_false((td->td_dbgflags & TDB_USERWR) != 0)) {
+			/*
+			 * Reread syscall number and arguments if debugger
+			 * modified registers or memory.
+			 */
+			error = (p->p_sysent->sv_fetch_syscall_args)(td);
 #ifdef KTRACE
-		if (KTRPOINT(td, KTR_SYSCALL))
-			ktrsyscall(sa->code, sa->narg, sa->args);
+			if (KTRPOINT(td, KTR_SYSCALL))
+				ktrsyscall(sa->code, sa->narg, sa->args);
 #endif
-		if (error != 0) {
-			td->td_errno = error;
-			goto retval;
+			if (error != 0) {
+				td->td_errno = error;
+				goto retval;
+			}
 		}
 	}