Re: [Intel-gfx] [4.2-rc4] acpi|drm|i915: circular locking dependency: acpi_video_get_backlight_type

[Hans de Goede]

Hi,

On 13-08-15 16:33, Hans de Goede wrote:
> Hi,
>
> On 12-08-15 21:26, Ville Syrjälä wrote:
> > On Mon, Aug 10, 2015 at 08:29:00PM +0200, Sedat Dilek wrote:
> > > On Sat, Aug 1, 2015 at 2:23 PM, Sedat Dilek <sedat.dilek@xxxxxxxxx> wrote:
> > > > On Mon, Jul 27, 2015 at 12:33 AM, Sedat Dilek <sedat.dilek@xxxxxxxxx> wrote:
> > > > > Hi,
> > > > >
> > > > > this my first build of a 4.2-rcN Linux-kernel and I see this...
> > > >
> > > > Just FYI:
> > > >
> > > > I am *not* seeing this with drm-intel-nightly from below url.
> > > >
> > > > Also, I plan to test Linux v4.2-rc5.
> > >
> > > [ CC Linus ]
> > >
> > > Knock Knock Knock.
> > >
> > > This issue still remains here (with CONFIG_DRM_I915=m)...
> > >
> > > [   18.269792] ======================================================
> > > [   18.269798] [ INFO: possible circular locking dependency detected ]
> > > [   18.269805] 4.2.0-rc6-1-iniza-small #1 Not tainted
> > > [   18.269810] -------------------------------------------------------
> > > [   18.269816] modprobe/727 is trying to acquire lock:
> > > [   18.269822]  (init_mutex){+.+.+.}, at: [<ffffffffa0090f2d>]
> > > acpi_video_get_backlight_type+0x17/0x164 [video]
> > > [   18.269840]
> > > [   18.269840] but task is already holding lock:
> > > [   18.269848]  (&(&backlight_notifier)->rwsem){++++..}, at:
> > > [<ffffffff810a6519>] __blocking_notifier_call_chain+0x39/0x70
> > > [   18.269864]
> > > [   18.269864] which lock already depends on the new lock.
> > > [   18.269864]
> > > [   18.269875]
> > > [   18.269875] the existing dependency chain (in reverse order) is:
> > > [   18.269884]
> > > ...
> > >
> > > Full dmesg log and kernel-config attached.
> > >
> > > Shall I add Rusty and modules/modprobe folks?
> >
> > Just got back from vacation and was greeted by this same lockdep splat.
> >
> > On a hunch I reverted
> >
> > commit 93a291dfaf9c328ca5a9cea1733af1a128efe890
> > Author: Hans de Goede <hdegoede@xxxxxxxxxx>
> > Date:   Tue Jun 16 16:27:52 2015 +0200
> >
> >      ACPI / video: Move backlight notifier to video_detect.c
> >
> > and the problem seems to be gone. Hans, any thoughts?
>
> Looking into this atm, lockdep clearly is right.
>
> Sorry about this I have put a lot of thinking into avoiding
> these kind of issues with this patch-set, but I did not realize
> there was another lock "hiding" inside the notifier-chain.
>
> Further analysis shows that the lock inside the notifier-chain
> causes similar problems vs register_count_mutex from
> drivers/acpi/acpi_video.c. I'm working on a fix for this
> atm.

Heh, look at what I just found (I'm shifting my work focus
in the direction of nouveau) :

https://bugzilla.redhat.com/show_bug.cgi?id=1152876

So it looks like we have had the root cause of this issue
for a long time already, maybe my recent backlight selection
logic cleanup / rework has made it easier to trigger the
lockdep warning for this though.

Anyhow assuming people are ok with the fix I submitted
yesterday we've a fix for this now.

Regards,

Hans
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