Re: [PATCH 1/1][v2] Add reboot_pid_ns to handle the reboot syscall

[Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

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.