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

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Message-ID: <4ECFC5E3.5090900@parallels.com>
Date:	Fri, 25 Nov 2011 20:44:19 +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 08:22 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.

>>  * one more field on struct pid is OK, since it size doesn't change (32 bit level is
>>    anyway not required, it's OK to reduce on down to 16 bits)
> 
> Even if sizeof is the same, the new member and the code which plays
> with ->flags doesn't make the things better ;)
> 
>>  * yes, we have +1 member on task_struct :(
> 
> 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 :) But the code complication is the problem, yes :(

>> Oleg, Tejun, do you agree with such an approach?
> 
> If set_last_pid doesn't work, I'd prefer CLONE_CHILD_USEPIDS.

OK, thanks.

> Oleg.
> 
> .
> 

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