[src/trunk]: src/sys/kern Fix one the the root causes of unreliability of the...

[kamil <kamil%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/2b43b062837d
branches:  trunk
changeset: 966069:2b43b062837d
user:      kamil <kamil%NetBSD.org@localhost>
date:      Sun Oct 13 03:19:57 2019 +0000

description:
Fix one the the root causes of unreliability of the ptrace(2)ed threads

In case of sigswitchin away in issignal() and continuing the execution on
PT_CONTINUE (or equivalent call), there is a time window when another
thread could cause the process state to be changed to PS_STOPPING.

In the current logic, a thread would receive signal 0 (no-signal) and exit
from issignal(), returning to userland and never finishing the process of
stopping all LWPs. This causes hangs waitpid() waiting for SIGCHLD and
the callout polling for the state of the process in an infinite loop.

Instead of prompting for a returned signal from a debugger, repeat the
issignal() loop, this will cause checking the PS_STOPPING flag again and
sigswitching away in the scenario of stopping the process.

diffstat:

 sys/kern/kern_sig.c |  6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)

diffs (27 lines):

diff -r 8b0934132143 -r 2b43b062837d sys/kern/kern_sig.c
--- a/sys/kern/kern_sig.c       Sun Oct 13 03:10:22 2019 +0000
+++ b/sys/kern/kern_sig.c       Sun Oct 13 03:19:57 2019 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: kern_sig.c,v 1.370 2019/10/13 03:10:22 kamil Exp $     */
+/*     $NetBSD: kern_sig.c,v 1.371 2019/10/13 03:19:57 kamil Exp $     */
 
 /*-
  * Copyright (c) 2006, 2007, 2008 The NetBSD Foundation, Inc.
@@ -70,7 +70,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: kern_sig.c,v 1.370 2019/10/13 03:10:22 kamil Exp $");
+__KERNEL_RCSID(0, "$NetBSD: kern_sig.c,v 1.371 2019/10/13 03:19:57 kamil Exp $");
 
 #include "opt_ptrace.h"
 #include "opt_dtrace.h"
@@ -1865,7 +1865,7 @@
                if (p->p_stat == SSTOP || (p->p_sflag & PS_STOPPING) != 0) {
                        sigswitch_unlock_and_switch_away(l);
                        mutex_enter(p->p_lock);
-                       signo = sigchecktrace();
+                       continue;
                } else if (p->p_stat == SACTIVE)
                        signo = sigchecktrace();
                else