Re: bug in memcg oom-killer results in a hung syscall in another process in the same cgroup

[Oleg Nesterov <oleg@redhat.com>]

On 07/12, Shayan Pooya wrote:
>
> > Yep. Bug still not fixed in upstream. In our kernel I've plugged it with
> > this:
> >
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -2808,8 +2808,9 @@ asmlinkage __visible void schedule_tail(struct
> > task_struct *prev)
> >         balance_callback(rq);
> >         preempt_enable();
> >
> > -       if (current->set_child_tid)
> > -               put_user(task_pid_vnr(current), current->set_child_tid);
> > +       if (current->set_child_tid &&
> > +           put_user(task_pid_vnr(current), current->set_child_tid))
> > +               force_sig(SIGSEGV, current);
> >  }
>
> I just verified that with your patch there is no hung processes and I
> see processes getting SIGSEGV as expected.

Well, but we can't do this. And "as expected" is actually just wrong. I still
think that the whole FAULT_FLAG_USER logic is not right. This needs another email.

fork() should not fail because there is a memory hog in the same memcg. Worse,
pthread_create() can kill the caller by the same reason. And we have the same
or even worse problem with ->clear_child_tid, pthread_join() can hang forever.
Unlikely we want to kill the application in this case ;)

And in fact I think that the problem has nothing to do with set/claer_child_tid
in particular.

I am just curious... can you reproduce the problem reliably? If yes, can you try
the patch below ? Just in case, this is not the real fix in any case...

Oleg.

--- x/kernel/sched/core.c
+++ x/kernel/sched/core.c
@@ -2793,8 +2793,11 @@ asmlinkage __visible void schedule_tail(struct task_struct *prev)
 	balance_callback(rq);
 	preempt_enable();
 
-	if (current->set_child_tid)
+	if (current->set_child_tid) {
+		mem_cgroup_oom_enable();
 		put_user(task_pid_vnr(current), current->set_child_tid);
+		mem_cgroup_oom_disable();
+	}
 }
 
 /*