linux-kernel - Re: Atomicity in KMS panic notifier

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Date:	Tue, 06 May 2014 15:38:47 +0200
From:	Takashi Iwai <tiwai@...e.de>
To:	David Herrmann <dh.herrmann@...il.com>
Cc:	Daniel Vetter <daniel@...ll.ch>,
	dri-devel <dri-devel@...ts.freedesktop.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Atomicity in KMS panic notifier

At Tue, 6 May 2014 15:32:21 +0200,
David Herrmann wrote:
> 
> Hi
> 
> On Tue, May 6, 2014 at 3:27 PM, Takashi Iwai <tiwai@...e.de> wrote:
> > At Mon, 5 May 2014 16:52:45 +0200,
> > Daniel Vetter wrote:
> >>
> >> On Mon, May 5, 2014 at 4:48 PM, Takashi Iwai <tiwai@...e.de> wrote:
> >> >
> >> > The current problem I see is that the rest of panic notifier chain
> >> > won't be called once when we hit the problem in KMS notifier.  So,
> >> > this bug in KMS influences on the rest panic behavior.
> >> >
> >> > Maybe a hackish solution would be to keep KMS notifier at the end of
> >> > notifier chain so that it crashes at last.  I don't like this either,
> >> > but...
> >>
> >> You need to do that with both the kms panic notifier in
> >> drm_fb_helper.c and with the fbcon panic notifier. And iirc there's
> >> also a console->unblank call somewhere which _also_ can end up in
> >> ->set_par. But I'm not sure anymore when exactly that one is run, I've
> >> tried hard to forget this all ;-)
> >
> > Looking back at the code again, it seems that fbcon has no panic
> > notifier.  It has own notifier chain, but it's a private chain that
> > isn't called by the panic.  So, we can forget about fbcon, at least (I
> > hope).
> 
> fbcon is called through the VT or fbdev layer, which is called by
> bust_spinlocks(1) via either unblank_screen() or console_unblank().

You mean bust_spinlocks(0), right?

void __attribute__((weak)) bust_spinlocks(int yes)
{
	if (yes) {
		++oops_in_progress;
	} else {
#ifdef CONFIG_VT
		unblank_screen();
#endif
		console_unblank();
		if (--oops_in_progress == 0)
			wake_up_klogd();
	}
}

bust_spinlocks(0) is called after the notifier chain, and it's almost
at the end of panic().


Takashi
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