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Date: Wed, 11 Apr 2007 23:49:21 +0400
From: Oleg Nesterov <oleg@tv-sign.ru>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
       Davide Libenzi <davidel@xmailserver.org>,
       Jan Engelhardt <jengelh@linux01.gwdg.de>, Ingo Molnar <mingo@elte.hu>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Robin Holt <holt@sgi.com>, Roland McGrath <roland@redhat.com>,
       "Serge E. Hallyn" <serge@hallyn.com>, linux-kernel@vger.kernel.org,
       "Rafael J. Wysocki" <rjw@sisk.pl>, Gautham R Shenoy <ego@in.ibm.com>
Subject: Re: [PATCH] kthread: Don't depend on work queues (take 2)
Message-ID: <20070411194921.GA171@tv-sign.ru>
References: <20070410185133.GA104@tv-sign.ru> <m13b37v517.fsf@ebiederm.dsl.xmission.com> <20070411000316.52f2551e.akpm@linux-foundation.org> <m1hcrmsr4g.fsf_-_@ebiederm.dsl.xmission.com> <20070411192809.GA106@tv-sign.ru> <m14pnmsnrs.fsf@ebiederm.dsl.xmission.com>
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On 04/11, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@tv-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.