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

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Message-ID: <20130131001339.GC14801@flint.arm.linux.org.uk>
Date:	Thu, 31 Jan 2013 00:13:40 +0000
From:	Russell King <rmk@....linux.org.uk>
To:	Dave Airlie <airlied@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Florian Tobias Schandinat <FlorianSchandinat@....de>,
	"linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: BUG: circular locking dependency detected

On Thu, Jan 31, 2013 at 10:04:05AM +1000, Dave Airlie wrote:
> On Thu, Jan 31, 2013 at 9:52 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> > On Thu, Jan 31, 2013 at 9:19 AM, Russell King <rmk@....linux.org.uk> wrote:
> >>
> >> 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...
> >
> > Oh, we're extremely aware of it. And it's not a new issue, the locking
> > problem have apparently been around forever, although I'm not sure why
> > the lockdep splat itself started happening only recently.
> >
> > They'll make it into 3.9, it's 3.8 that won't have them. The patches
> > initially caused way *worse* behavior than just a lockdep splat - they
> > caused actual hard lockups (and that was *after* the initial series of
> > fixes). That got fixed (hopefully for the last case!) fairly recently,
> > and I'm not willing to take the scary patch-series that has had
> > several problem cases.
> 
> Well we didn't have any lock validation support before Daniel added it
> a couple of kernels back,
> so instead of hidden locking problems we've had from time began, we now have
> lockdep detectable locking problems.

Which may or may not be a good thing depending how you look at it; it
means that once your kernel blanks, you get a lockdep dump.  At that
point you lose lockdep checking for everything else because lockdep
disables itself after the first dump.

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