linux-kernel - Re: [RFC][PATCH 0/3] fork: Add the ability to create tasks with given pids

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Message-ID: <20111124173121.GA23260@redhat.com>
Date:	Thu, 24 Nov 2011 18:31:21 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Pavel Emelyanov <xemul@...allels.com>
Cc:	Tejun Heo <tj@...nel.org>, Pedro Alves <pedro@...esourcery.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	James Bottomley <jbottomley@...allels.com>
Subject: Re: [RFC][PATCH 0/3] fork: Add the ability to create tasks with
	given pids

On 11/24, Pavel Emelyanov wrote:
>
> Hm... Started testing this stuff and thought about Pedro's wish to use this
> in gdb one more time :(
>
> The thing is, that we (in checkpoint/restore) are going to use this sysctl
> when creating a pid namespace from scratch, thus having full control over
> all the forks happening in this namespace.
>
> But when it comes to the ability for gdb to create a task with a given pid in
> a _living_ namespace this solution simply won't work! It doesn't guarantee,
> that after setting the last_pid via sysctl this last_pid stays the same at
> the time we do call fork()/clone(). Because there are other tasks that can call
> fork themselves ignoring any lask_pid locking we can play with.

Sure. ->last_pid is a hint, nothing more.

But nothing can work reliably on the running system. Even if we can't
race with another fork(), the required pid_nr can be already in use.

Oleg.

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