Re: strace lockup when tracing exec in go

[Mike Galbraith <umgwanakikbuti@gmail.com>]

On Thu, 2016-09-22 at 10:36 +0200, Michal Hocko wrote:
> 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.
>  > 	>  */

This patch doesn't help, nor does the previous patch... but with both
applied, all is well.  All you have to do now is figure out why :)

	-Mike