linux-kernel - Re: [PATCH 3/3] pids: Make it possible to clone tasks with given pids

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Date:	Fri, 11 Nov 2011 17:39:26 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Pavel Emelyanov <xemul@...allels.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Glauber Costa <glommer@...allels.com>,
	Nathan Lynch <ntl@...ox.com>, Tejun Heo <tj@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Serge Hallyn <serue@...ibm.com>,
	Daniel Lezcano <dlezcano@...ibm.com>
Subject: Re: [PATCH 3/3] pids: Make it possible to clone tasks with given
	pids

On 11/11, Pavel Emelyanov wrote:
>
> On 11/11/2011 07:25 PM, Oleg Nesterov wrote:
> >
> > But. Let me repeat the question, what if you do the same with
> > pids[0] = 2 /* anything != 1 */ ? In this case we create the new
> > pid_ns, but its ->child_reaper is NULL. Unless I missed something.
>
> Hm... You're right here. I've missed the fact, then in recent kernels
> child_reaper is set under pid == 1 condition (was clone_flags & CLONE_NEWPID).

Yes, I always hated the "cleanup" which removed CLONE_NEWPID from
copy_process. This is_child_reaper() simply hides CLONE_NEWPID from
grep.

But this is offtopic. We should not create ->child_reaper with pid_nr != 1.

> How about if I fix it by disabling the simultaneous use of CLONE_NEWPID and
> CLONE_CHILD_USEPIDS and checking for last_pid != 1 in the set_pidmap?

I think this should work...

> > Hmm. It seems, we can make a simpler patch to achieve the (roughly)
> > same effect. Without touching copy_process/alloc_pid paths. What if
> > we simply add PR_SET_LAST_PID? (or something else).
> >
> > In this case the new init (created normally) read the pids from image
> > file and does prcrl(PR_SET_LAST_PID, pid-1) before the next fork.
> >
> > What do you think?
>
> This will make it impossible to fork() children on restore in parallel. And
> I don't want to lose this ability :(

Yes, this is true. You need some form of synchronization in user-space.
But, otoh, prctl/sysctl/whatever is much simpler. Both from implementation
pov and from understanding/using. You can even do, say, pthread_create()
to make a thread with the desired tid. And of course I like the fact we
do not add the new hacks into copy_process's paths.

And. If you want to restore the process tree, then these new children
have to cooperate anyway. Say, nobody can clone() without
CLONE_CHILD_USEPIDS before we restore all pids.

Yes, sysctl+clone should be "atomic", but that is all. Does it really
hurt? OK, if nothing else, can't you do somthing like

	int fork_with_pid(int pid)
	{
		int ret;
		int pipefd[2];

		pipe(pipefd);

	retry:
		prcrl(PR_SET_LAST_PID, pid-1);
		ret = fork();

		if (ret == 0) {
			/* child, wait from parent's ACK */
			read(pipefd[0], 1, NULL);
			return 0;
		}

		/* raced with another user of PR_SET_LAST_PID */
		if (unlikely(ret != pid) {
			kill(ret, SIGKILL);
			waitpid(ret);
			goto retry;
		}

		close(pipefd[1]);
		return pid;
	}

?

Oleg.

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