From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Subject: Re: [PATCH] ACPI / video: Fix circular lock dependency issue in the video-detect code
Date: Fri, 14 Aug 2015 12:16:37 +0200
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To: sedat.dilek@gmail.com
Cc: Hans de Goede <hdegoede@redhat.com>, Aaron Lu <aaron.lu@intel.com>, Ville =?ISO-8859-1?Q?Syrj=E4l=E4?= <ville.syrjala@linux.intel.com>, Daniel Vetter <daniel.vetter@ffwll.ch>, Zhang Rui <rui.zhang@intel.com>, Len Brown <lenb@kernel.org>, Linus Torvalds <torvalds@linux-foundation.org>, Linux ACPI <linux-acpi@vger.kernel.org>, intel-gfx <intel-gfx@lists.freedesktop.org>

On Friday, August 14, 2015 11:36:41 AM Sedat Dilek wrote:
> On Fri, Aug 14, 2015 at 11:39 AM, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> > On Thursday, August 13, 2015 10:39:08 PM Sedat Dilek wrote:
> >>
> >> --f46d04447e7fc2306e051d3753a5
> >> Content-Type: text/plain; charset=UTF-8
> >>
> >> On Thu, Aug 13, 2015 at 6:53 PM, Hans de Goede <hdegoede@redhat.com> wrote:
> >> > Before this commit, the following would happen:
> >> >
> >> > a) acpi_video_get_backlight_type() gets called
> >> > b) acpi_video_get_backlight_type() calls acpi_video_init_backlight_type()
> >> > c) acpi_video_init_backlight_type() locks its function static init_mutex
> >> > d) acpi_video_init_backlight_type() calls backlight_register_notifier()
> >> > e) backlight_register_notifier() takes its notifier-chain lock
> >> >
> >> > And when the backlight notifier chain gets called we've:
> >> >
> >> > 1) blocking_notifier_call_chain() gets called
> >> > 2) blocking_notifier_call_chain() takes the notifier-chain lock
> >> > 3) blocking_notifier_call_chain() calls acpi_video_backlight_notify()
> >> > 4) acpi_video_backlight_notify() calls acpi_video_get_backlight_type()
> >> > 5) acpi_video_get_backlight_type() calls acpi_video_init_backlight_type()
> >> > 6) acpi_video_init_backlight_type() locks its function static init_mutex
> >> >
> >> > So in the first call sequence we have:
> >> >
> >> > a) init_mutex gets locked
> >> > b) notifier-chain gets locked
> >> >
> >> > and in the second call sequence we have:
> >> >
> >> > 1) notifier-chain gets locked
> >> > 2) init_mutex gets locked
> >> >
> >> > And we've a circular locking dependency. This specific locking dependency
> >> > is fixable without using the big hammer otherwise known as a workqueue,
> >> > but further analysis shows a similar problem with the backlight notifier
> >> > chain lock vs register_count_mutex from drivers/acpi/acpi_video.c,
> >> > and fixing that becomes problematic.
> >> >
> >> > So this commit simply fixes this with the big hammer, performance
> >> > wise this is a non issue as we expect the work to get scheduled
> >> > exactly zero or one times during normal system use.
> >> >
> >>
> >> This patch on top of Linux v4.2-rc6 fixes the issue for me.
> >>
> >> Feel free to add my...
> >>
> >>    Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
> >>    Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
> >
> > Applied, thanks!
> >
> 
> Thanks for carrying this one.
> Isn't this a for-4.2/acpi-video-fixes?

I have no branch like that.

> Why did you apply this in your linux-next Git branch?

Because that's what I do normally with patches I'm going to push to Linus.

Thanks,
Rafael