linux-kernel - Re: [PATCH -mm v2 1/3] mm/oom_kill: remove the wrong fatal_signal_pending() check in oom_kill

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Date:	Fri, 2 Oct 2015 15:32:44 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	oleg@...hat.com, akpm@...ux-foundation.org, rientjes@...gle.com,
	kwalker@...hat.com, skozina@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm v2 1/3] mm/oom_kill: remove the wrong
 fatal_signal_pending() check in oom_kill_process()

On Fri 02-10-15 21:33:08, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > > Since T sends SIGKILL to all clone(CLONE_VM) tasks upon coredump, P needs
> > > to do
> > 
> > It does that only to all threads in the _same_ thread group AFAIU.
> 
> I'm confused. What the _same_ thread group?
> 
> I can observe that SIGKILL is sent to all
> 
>   clone(CLONE_THREAD | CLONE_SIGHAND | CLONE_VM)
>   clone(CLONE_SIGHAND | CLONE_VM)
>   clone(CLONE_VM)

I might be missing something crucial here but
copy_process has the following:
        if (clone_flags & CLONE_THREAD) {
                p->exit_signal = -1;
                p->group_leader = current->group_leader;
                p->tgid = current->tgid;
        } else {
                if (clone_flags & CLONE_PARENT)
                        p->exit_signal = current->group_leader->exit_signal;
                else
                        p->exit_signal = (clone_flags & CSIGNAL);
                p->group_leader = p;
                p->tgid = p->pid;
        }

So clone without CLONE_THREAD should create a new thread group leader
and so create a new thread group. Unless there is some other trickery
which I do not see right now for_each_thread from the parent task
shouldn't see those which are cloned without CLONE_THREAD.

[...]
> --- a/fs/coredump.c
> +++ b/fs/coredump.c
> @@ -295,6 +295,8 @@ static int zap_process(struct task_struct *start, int exit_code, int flags)
>         for_each_thread(start, t) {
>                 task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK);
>                 if (t != current && t->mm) {
> +                       printk(KERN_INFO "Setting SIGKILL to %s(%u)\n",
> +                              t->comm, t->pid);
>                         sigaddset(&t->pending.signal, SIGKILL);
>                         signal_wake_up(t, 1);
>                         nr++;
> ---------- debug printk() patch end ----------

OK, but all your tasks should trigger SEGV. You cannot find out whether
all of them happened from the same zap_process, can you.
-- 
Michal Hocko
SUSE Labs
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