Re: strace lockup when tracing exec in go

[Michal Hocko]

On Thu 22-09-16 10:01:26, Michal Hocko wrote:
> On Thu 22-09-16 06:15:02, Mike Galbraith wrote:
> [...]
> > master.today...
> 
> Thanks for trying to reproduce this. My tiny laptop (2 cores, 2 threads
> per core) cannot reproduce even in 10 minutes or so. I've tried to use
> the same machine I was testing with 3.12 kernel (2 sockets, 8 cores per
> soc. and 2 threas per core) and it hit almost instantly. I have tried 
> mutex_lock_killable -> interruptible and it didn't help as I've
> expected. So the current kernel doesn't do any magic to prevent from the
> issue as well.
> 
> So I've stared into do_notify_parent some more and the following was
> just very confusing
> 
> 	if (!tsk->ptrace && sig == SIGCHLD &&
> 	    (psig->action[SIGCHLD-1].sa.sa_handler == SIG_IGN ||
> 	     (psig->action[SIGCHLD-1].sa.sa_flags & SA_NOCLDWAIT))) {
> 		/*
> 		 * We are exiting and our parent doesn't care.  POSIX.1
> 		 * defines special semantics for setting SIGCHLD to SIG_IGN
> 		 * or setting the SA_NOCLDWAIT flag: we should be reaped
> 		 * automatically and not left for our parent's wait4 call.
> 		 * Rather than having the parent do it as a magic kind of
> 		 * signal handler, we just set this to tell do_exit that we
> 		 * can be cleaned up without becoming a zombie.  Note that
> 		 * we still call __wake_up_parent in this case, because a
> 		 * blocked sys_wait4 might now return -ECHILD.
> 		 *
> 		 * Whether we send SIGCHLD or not for SA_NOCLDWAIT
> 		 * is implementation-defined: we do (if you don't want
> 		 * it, just use SIG_IGN instead).
> 		 */
> 		autoreap = true;
> 		if (psig->action[SIGCHLD-1].sa.sa_handler == SIG_IGN)
> 			sig = 0;
> 	}
> 
> it tries to prevent from what I am seeing in a way. If the SIGCHLD is
> ignored then it just does autoreap and everything is fine. But this
> doesn't seem to be the case here. In fact we are not sending the signal
> because sig_task_ignored is true resp. sig_handler_ignored which can
> fail even for handler == SIG_DFL && sig_kernel_ignore() and SIGCHLD
> seems to be in SIG_KERNEL_IGNORE_MASK. So I've tried

Dohh, I've missed !tsk->ptrace check there. So we are not even going
that via that path. So the sig_handler_ignored cannot possible help.
I was just too lucky and didn't hit the lockup with the patch.

So what else might be wrong here? sig_ignored seems to be quite
confusing

	/*
	 * Tracers may want to know about even ignored signals.
	 */
	return !t->ptrace;

t is the tracer here but it shouldn't have t->ptrace because the child
is not stopped. So do we need something like the following? Not tested
yet

diff --git a/kernel/signal.c b/kernel/signal.c
index 1840c7f4e3c2..bd236ce4a29c 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -91,6 +91,10 @@ static int sig_ignored(struct task_struct *t, int sig, bool force)
 	if (!sig_task_ignored(t, sig, force))
 		return 0;
 
+	/* Do not ignore signals sent from child to the parent */
+	if (current->ptrace && current->parent == t)
+		return 0;
+
 	/*
 	 * Tracers may want to know about even ignored signals.
 	 */
-- 
Michal Hocko
SUSE Labs