linux-kernel - Re: [PATCH] kthread: Don't depend on work queues (take 2)

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Message-ID: <20070411194921.GA171@tv-sign.ru>
Date:	Wed, 11 Apr 2007 23:49:21 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Davide Libenzi <davidel@...ilserver.org>,
	Jan Engelhardt <jengelh@...ux01.gwdg.de>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Robin Holt <holt@....com>, Roland McGrath <roland@...hat.com>,
	"Serge E. Hallyn" <serge@...lyn.com>, linux-kernel@...r.kernel.org,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Gautham R Shenoy <ego@...ibm.com>
Subject: Re: [PATCH] kthread: Don't depend on work queues (take 2)

On 04/11, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@...sign.ru> writes:
> 
> > On 04/11, Eric W. Biederman wrote:
> >> 
> >> @@ -435,8 +436,12 @@ static void __init setup_command_line(char *command_line)
> >>  static void noinline rest_init(void)
> >>  	__releases(kernel_lock)
> >>  {
> >> +	int pid;
> >>  	kernel_thread(init, NULL, CLONE_FS | CLONE_SIGHAND);
> >>  	numa_default_policy();
> >> +
> >> +	pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
> >> +	kthreadd_task = find_task_by_pid(pid);
> >>  	unlock_kernel();
> >
> > Just curious. What if kernel/kthread.c declares
> >
> > 	static struct task_struct *kthreadd_task = &init_task;
> >
> > an then kthreadd_setup() does kthreadd_task = current. I assume it is always
> > safe to try_to_wake_up(idle_thread), because it always TASK_RUNNING. This
> > way we don't need to export kthreadd_task.
> 
> I did it this way largely so I could use the export in reparent_to_XXX in
> daemonize.  This way I don't have races in finding kthreadd.  Plus
> I didn't think of the trick of using the idle_thread...

Ah yes, we still need to export kthreadd_task... I just worried about subtle
dependency this patch adds... This "kthreadd_task = find_task_by_pid" assumes
that init/main.c:init() takes lock_kernel() before the first kthread_create().

Oleg.

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