[Intel-gfx] [PATCH] ACPI / video: Fix circular lock dependency issue in the video-detect code

[Sedat Dilek]

On Fri, Aug 14, 2015 at 12:16 PM, Rafael J. Wysocki <rjw at rjwysocki.net> wrote:
> 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 at 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 at 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 at gmail.com>
>> >>    Tested-by: Sedat Dilek <sedat.dilek at 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.
>

I wanted to say it's "for 4.2-rc7".

>> 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.
>

OK, thanks.

- sed@ -

> Thanks,
> Rafael
>