linux-kernel - Re: [PATCH 2/3] make kthread

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Message-ID: <m1lkgwndsu.fsf@ebiederm.dsl.xmission.com>
Date:	Fri, 13 Apr 2007 15:51:29 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Oleg Nesterov <oleg@...sign.ru>,
	Davide Libenzi <davidel@...ilserver.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Roland McGrath <roland@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] make kthread_create() more scalable

Andrew Morton <akpm@...ux-foundation.org> writes:

> On Fri, 13 Apr 2007 17:02:01 +0400
> Oleg Nesterov <oleg@...sign.ru> wrote:
>
>> If kernel_thread(kthread) succeeds, kthread() can not fail on its path to
>> complete(&create->started) + schedule(). After that it can't be woken because
>> nobody can see the new task yet. This means:
>> 
>> 	- we don't need tasklist_lock for find_task_by_pid().
>> 
>> 	- create_kthread() doesn't need to wait for create->started. Instead,
>> 	  kthread_create() first waits for create->created to get the result of
>> 	  kernel_thread(), then waits for create->started to synchronize with
>> 	  kthread().
>
> Why don't we need tasklist_lock for find_task_by_pid()?  I'd have though that
> we'd at least need rcu_read_lock(), and I'm not sure that the implicit
> understanding of pid-management internals here is a great idea.

We need rcu_read_lock().  Or else something could permute the pid hash table
and get us into trouble.

Eric





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