On Fri, 13 Apr 2007 15:51:29 -0600
[email protected] (Eric W. Biederman) wrote:
> Andrew Morton <[email protected]> writes:
>
> > On Fri, 13 Apr 2007 17:02:01 +0400
> > Oleg Nesterov <[email protected]> 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.
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