Index: sys/kern/sys_ptrace_common.c
===================================================================
RCS file: /cvsroot/src/sys/kern/sys_ptrace_common.c,v
retrieving revision 1.80
diff -u -r1.80 sys_ptrace_common.c
--- sys/kern/sys_ptrace_common.c	14 May 2020 13:32:15 -0000	1.80
+++ sys/kern/sys_ptrace_common.c	16 May 2020 11:44:19 -0000
@@ -1421,16 +1421,13 @@
 		 * the requested thread, and clear it for other threads.
 		 */
 		LIST_FOREACH(lt2, &t->p_lwps, l_sibling) {
-			if (ISSET(lt2->l_pflag, LP_SINGLESTEP)) {
-				lwp_lock(lt2);
-				process_sstep(lt2, 1);
-				lwp_unlock(lt2);
-			} else if (lt != lt2) {
-				lwp_lock(lt2);
-				process_sstep(lt2, 0);
-				lwp_unlock(lt2);
-			}
+			error = process_sstep(lt2,
+			    ISSET(lt2->l_pflag, LP_SINGLESTEP));
+			if (error)
+				break;
 		}
+		if (error)
+			break;
 		error = process_sstep(lt,
 		    ISSET(lt->l_pflag, LP_SINGLESTEP) || req == PT_STEP);
 		if (error)