linux-kernel - Re: [PATCH] driver core: add wait event for deferred probe

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Date:	Thu, 14 Feb 2013 23:52:14 +0800
From:	Haojian Zhuang <haojian.zhuang@...aro.org>
To:	Grant Likely <grant.likely@...retlab.ca>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	viro@...iv.linux.org.uk, rusty@...tcorp.com.au,
	hpa@...ux.intel.com, jim.cromie@...il.com,
	linux-kernel@...r.kernel.org,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Linus Walleij <linus.walleij@...aro.org>, arnd@...db.de,
	broonie@...nsource.wolfsonmicro.com,
	Patch Tracking <patches@...aro.org>
Subject: Re: [PATCH] driver core: add wait event for deferred probe

On 14 February 2013 05:36, Grant Likely <grant.likely@...retlab.ca> wrote:
> On Tue, 12 Feb 2013 10:52:10 +0800, Haojian Zhuang <haojian.zhuang@...aro.org> wrote:
>> On 12 February 2013 07:10, Andrew Morton <akpm@...ux-foundation.org> wrote:
>> > On Sun, 10 Feb 2013 00:57:57 +0800
>> > Haojian Zhuang <haojian.zhuang@...aro.org> wrote:
>> >
>> >> do_initcalls() could call all driver initialization code in kernel_init
>> >> thread. It means that probe() function will be also called from that
>> >> time. After this, kernel could access console & release __init section
>> >> in the same thread.
>> >>
>> >> But if device probe fails and moves into deferred probe list, a new
>> >> thread is created to reinvoke probe. If the device is serial console,
>> >> kernel has to open console failure & release __init section before
>> >> reinvoking failure. Because there's no synchronization mechanism.
>> >> Now add wait event to synchronize after do_initcalls().
>> >
>> > It sounds like this:
>> >
>> > static int __ref kernel_init(void *unused)
>> > {
>> >         kernel_init_freeable();
>> >         /* need to finish all async __init code before freeing the memory */
>> >         async_synchronize_full();
>> >
>> > is designed to prevent the problem you describe?
>> >
>> It can't prevent the problem that I described. Because deferred_probe()
>> is introduced recently.
>>
>> All synchronization should be finished just after do_initcalls(). Since
>> load_default_modules() is also called in the end of kernel_init_freeable(),
>> I'm not sure that whether I could remove async_synchronize_full()
>> here. So I didn't touch it.
>>
>> >> --- a/init/main.c
>> >> +++ b/init/main.c
>> >> @@ -786,6 +786,7 @@ static void __init do_basic_setup(void)
>> >>       do_ctors();
>> >>       usermodehelper_enable();
>> >>       do_initcalls();
>> >> +     wait_for_device_probe();
>> >>  }
>> >
>> > Needs a nice comment here explaining what's going on.
>>
>> No problem. I'll add comment here.
>
> Actually, this approach will create new problems. There is no guarantee
> that a given device will be able to initialize before exiting
> do_basic_setup(). If, for instance, a device depends on a resource
> provided by a module, then it will just keep deferring. In that case
> you've got a hung kernel.
>
> I think what you really want is the following:
>
>  static int deferred_probe_initcall(void)
>  {
>         deferred_wq = create_singlethread_workqueue("deferwq");
>         if (WARN_ON(!deferred_wq))
>                 return -ENOMEM;
>
>         driver_deferred_probe_enable = true;
> +       deferred_probe_work_func(NULL);
> -       driver_deferred_probe_trigger();

If you can change it into code in below, it could work. Otherwise, it
always fails.
        driver_deferred_probe_enable = true;
        driver_deferred_probe_trigger();
+      deferred_probe_work_func(NULL);
        return 0;

Because deferred_probe_work_func() depends on that deferred_probe is added
into deferred_probe_active_list. If driver_deferred_probe_trigger() isn't called
first, the deferred uart probe can't be added into active list. So even you call
work_func at here, it doesn't help.

Would you send it again? If so, you can add tested-by with my signature.

>         return 0;
>  }
>  late_initcall(deferred_probe_initcall);
>
> Or something similar. That would guarantee that as many passes as are needed
> (which in practical terms only means a couple) for device probing to
> settle down before exiting the initcall processing. That should achieve
> the effect you desire.
>
> It still masks the __init section issue by making it a lot less likely,
> but it does ensure that all of the built-in driver dependency order
> issues are processed before continuing on to userspace.
>
> g.
>
> --
> Grant Likely, B.Sc, P.Eng.
> Secret Lab Technologies, Ltd.
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