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

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Date:	Sun, 4 Dec 2011 16:45:43 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Daniel Lezcano <daniel.lezcano@...e.fr>
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, 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 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.

Oleg.

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