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Date:   Thu, 11 Mar 2021 16:08:21 +0100
From:   Oleg Nesterov <oleg@redhat.com>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Jim Newsome <jnewsome@torproject.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Christian Brauner <christian@brauner.io>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] do_wait: make PIDTYPE_PID case O(1) instead of O(n)
Message-ID: <20210311150820.GA15552@redhat.com>
References: <20210309203919.15920-1-jnewsome@torproject.org>
 <m1blbqmy2u.fsf@fess.ebiederm.org>
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On 03/10, Eric W. Biederman wrote:
>
> Jim Newsome <jnewsome@torproject.org> writes:
>
> > +static int do_wait_pid(struct wait_opts *wo)
> > +{
> > +	struct task_struct *target = pid_task(wo->wo_pid, PIDTYPE_PID);
>                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> This is subtle change in behavior.
>
> Today on the task->children list we only place thread group leaders.

Aaah, yes, thanks Eric!

> So the code either needs a thread_group_leader filter on target before
> the ptrace=0 case or we need to use "pid_task(wo->wo_pid, PIDTYPE_TGID)"
> and "pid_task(wo->wo_pid, PIDTYPE_PID)" for the "ptrace=1" case.

Agreed,

> I would like to make thread_group_leaders go away

Hmm, why?

Oleg.