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

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Date:	Fri, 25 Nov 2011 21:03:26 +0400
From:	Pavel Emelyanov <xemul@...allels.com>
To:	Oleg Nesterov <oleg@...hat.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 11/25/2011 08:54 PM, Oleg Nesterov wrote:
> On 11/25, Pavel Emelyanov wrote:
>>
>> On 11/25/2011 08:22 PM, Oleg Nesterov wrote:
>>> On 11/25, Pavel Emelyanov wrote:
>>>>
>>>> The proposal is to implement the PR_RESERVE_PID prctl which allocates and puts a
>>>> pid on the current. The subsequent fork() uses this pid,
>>>
>>> Oh. This is subjective, yes, but this doesn't clean to me.
>>>
>>> Amd why?? On the running system PR_RESERVE_PID can obviously fail anyway.
>>> It only helps to avoid the race with another fork.
>>
>> No. It can fail if you try to allocate a pid with given number. The API allows for
>> pid generation. AFAIU this can help with Pedro's requirements to resurrect task with
>> the same pid value it used to have before.
> 
> Yes gdb can do fork() in a row (until it unreserves the pid) and the
> pid will be the same.
> 
> OK, I misunderstood. I thought you insist that PR_RESERVE_PID itself
> is reliable.
> 
> But this can only work in the simplest case.

Yup!

> How you can restore the multithread tracee?

Don't know :) But if this approach sounds promising (I see, that now it's not, but...) I
can think more on it.

> You need to unreserve/reserve the previous pid, and we have the same problems again, no?

With the existing patch - yes, but as I said above - we need to decide which direction to
go and then I'll think further.

By now your opinion is to better stay where we are ;) but if moving is unavoidable, then
it's better to take the CLONE_CHILD_USEPIDS route. That's my position as well.

>>> Yes, and this task_struct->rsv_pid acts as implicit parameter for the
>>> next clone(). Doesn't look very nice to me. Plus the code complications.
>>
>> Well, the last_pid is also an implicit parameter for the next clone() with sysctl
>> approach :)
> 
> Yes. but it is already here ;)
> 
> Oleg.
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