Re: [PATCH v2 2/2] prctl: propagate has_child_subreaper flag to every descendant

[Pavel Tikhomirov]

On 01/30/2017 03:51 PM, Oleg Nesterov wrote:
> On 01/27, Pavel Tikhomirov wrote:
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -1725,6 +1725,8 @@ struct task_struct {
> >  	struct signal_struct *signal;
> >  	struct sighand_struct *sighand;
> >
> > +	struct list_head csr_descendant;
> > +
>
> You forgot to remove this part ;)

Oh, sure.

> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -1367,9 +1367,6 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
> >  	sig->oom_score_adj = current->signal->oom_score_adj;
> >  	sig->oom_score_adj_min = current->signal->oom_score_adj_min;
> >
> > -	sig->has_child_subreaper = current->signal->has_child_subreaper ||
> > -				   current->signal->is_child_subreaper;
> > -
> >  	mutex_init(&sig->cred_guard_mutex);
> >
> >  	return 0;
> > @@ -1800,6 +1797,13 @@ static __latent_entropy struct task_struct *copy_process(
> >
> >  			p->signal->leader_pid = pid;
> >  			p->signal->tty = tty_kref_get(current->signal->tty);
> > +			/*
> > +			 * Inherit has_child_subreaper flag under the same
> > +			 * tasklist_lock with adding child to the process tree
> > +			 * for propagate_has_child_subreaper optimization.
> > +			 */
> > +			p->signal->has_child_subreaper = current->signal->has_child_subreaper ||
> > +							 current->signal->is_child_subreaper;
>
> Ah yes, we need this change too...
>
> And perhaps it would be more correct to do
>
> 			p->signal->has_child_subreaper =
> 				p->real_parent->signal->has_child_subreaper ||
> 				p->real_parent->signal->is_child_subreaper;
>
> the current code is not exactly right if CLONE_PARENT...

I'm fine with inheriting 'has' flag from real_parent, because if 
real_parent does not have 'has' flag set but parent has 'has' set, we 
inherited the flag in vain.

But I don't actually think that inheritance from parent not real_parent 
breaks my optimization: if real_parent has the flag, so does the parent.

> Oleg.

--
Best regards, Tikhomirov Pavel
Software Developer, Virtuozzo.