linux-kernel - Re: [PATCH 1/1][v2] Add reboot_pid

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Date:	Sun, 04 Dec 2011 20:25:49 +0100
From:	Daniel Lezcano <daniel.lezcano@...e.fr>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	akpm@...ux-foundation.org, serge.hallyn@...onical.com,
	containers@...ts.linux-foundation.org, gkurz@...ibm.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1][v2] Add reboot_pid_ns to handle the reboot syscall

On 12/04/2011 04:45 PM, Oleg Nesterov wrote:
> On 12/04, Daniel Lezcano wrote:
>> @@ -32,6 +32,8 @@ struct pid_namespace {
>>  #endif
>>  	gid_t pid_gid;
>>  	int hide_pid;
>> +	int reboot;
>> +	spinlock_t reboot_lock;
>>  };
> Well. I was thinking about the serialization too, but this
> ->reboot_lock asks for v3 imho ;)
>
> First of all, do we really care? force_sig(SIGKILL, child_reaper)
> can't race with itself, it does nothing if init is already killed.
>
> So why it is important to protect pid_ns->reboot? Yes, it is possible
> to change it again if two callers do sys_reboot() "at the same time".
> But in this case we can't predict which caller wins anyway, so why
> should we worry?
>
> IOW. Say, we have the 2 tasks doing HALT and RESTART in parallel.
> It is possible that HALT sets ->reboot and kills init first, then
> RESTART changes ->reboot and the second force_sig() does nothing.
> In this case we can simply pretend that RESTART wins and the dying
> init kills HALT before it calls sys_reboot().

In the case of racy access, your argument makes sense but it is also to
prevent multiple calls to 'reboot'. In the init_pid_ns, when a shutdown
is on the way, the lock will prevent another task to invoke a machine
restart.  But anyway, we can get ride of this lock and the
serialization, it is a nit we can fix later if that makes sense with the
couple of lines you specified below.

> In any case. Even if you want to serialize, instead of adding the
> new lock reboot_pid_ns() can simply do:
>
> 	if (cmpxchg(&pid_ns->reboot, 0, reboot) != 0)
> 		return -EBUSY;
>
> this looks much simpler to me.

Yes, definitively :)

Thanks
  -- Daniel



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