linux-kernel - Re: BUG: circular locking dependency detected

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Message-ID: <20130130221946.GA14801@flint.arm.linux.org.uk>
Date:	Wed, 30 Jan 2013 22:19:46 +0000
From:	Russell King <rmk@....linux.org.uk>
To:	Daniel Vetter <daniel.vetter@...ll.ch>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Florian Tobias Schandinat <FlorianSchandinat@....de>,
	linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	dri-devel <dri-devel@...ts.freedesktop.org>,
	Dave Airlie <airlied@...il.com>
Subject: Re: BUG: circular locking dependency detected

On Wed, Jan 30, 2013 at 11:07:16PM +0100, Daniel Vetter wrote:
> On Wed, Jan 30, 2013 at 10:52 PM, Russell King <rmk@....linux.org.uk> wrote:
> > Also adding Greg and Daniel to this as Daniel introduced the lockdep
> > checking.
> >
> > This looks extremely horrid to be to solve - the paths are rather deep
> > where the dependency occurs.  The two paths between the locks are:
> >
> > console_lock+0x5c/0x70
> > register_con_driver+0x44/0x150
> > take_over_console+0x24/0x3b4
> > fbcon_takeover+0x70/0xd4
> > fbcon_event_notify+0x7c8/0x818
> > notifier_call_chain+0x4c/0x8c
> > __blocking_notifier_call_chain+0x50/0x68
> > blocking_notifier_call_chain+0x20/0x28
> >
> > and
> >
> > __blocking_notifier_call_chain+0x34/0x68
> > blocking_notifier_call_chain+0x20/0x28
> > fb_notifier_call_chain+0x20/0x28
> > fb_blank+0x40/0xac
> > fbcon_blank+0x1f4/0x29c
> > do_blank_screen+0x1b8/0x270
> > console_callback+0x74/0x138
> 
> You want Dave Airlie's pile of locking reworks, which fixes all
> currently known offenders around console_lock and fb_notifier. Patches
> won't go into 3.9 since it took a few rounds until they did not cause
> regression by making these deadlocks easier to hit.
> 
> http://cgit.freedesktop.org/~airlied/linux/log/?h=fbcon-locking-fixes
> 
> Long term solution would be to abolish the fb_notifier, at least for
> the purpose of linking fbdevs up with the fbcon and just replace those
> with direct function calls. But that requires that we no longer allow
> fbdev drivers and the fbcon to be loaded in any arbitrary order. Or
> just force fbcon to be built-in if enabled, imo the sane choice (no
> one's bothering with config_vt=m either, after all).

So... what you seem to be telling me is that 3.9 is going to be a
release which issues lockdep complaints when the console blanks, and
you think that's acceptable?

Adding Linus and Andrew so they're aware of this issue...

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:
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