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Date: Fri, 13 Apr 2007 15:08:41 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: ebiederm@xmission.com (Eric W. Biederman)
Cc: Oleg Nesterov <oleg@tv-sign.ru>, Davide Libenzi <davidel@xmailserver.org>,
       Ingo Molnar <mingo@elte.hu>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       "Rafael J. Wysocki" <rjw@sisk.pl>, Roland McGrath <roland@redhat.com>,
       Rusty Russell <rusty@rustcorp.com.au>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/3] make kthread_create() more scalable
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On Fri, 13 Apr 2007 15:51:29 -0600
ebiederm@xmission.com (Eric W. Biederman) wrote:

> Andrew Morton <akpm@linux-foundation.org> writes:
> 
> > On Fri, 13 Apr 2007 17:02:01 +0400
> > Oleg Nesterov <oleg@tv-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.
> 

OK, I fixed that up.

The next patch (make-kthread_stop-scalable) removes the find_task_by_pid()
anyway.

Our kthread creation performance will be pretty poor anyway, due to the
need to do two (or more?) context switches.  If we ever need
super-low-latency kernel thread creation (eg, on-demand threads for AIO)
then that code would need to go direct to kernel_thread(), I guess.