linux-kernel - Re: [REGRESSION] commit 66c1ca0: {fbmem: fix fb_info->lock and mm->mmap

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Date:	Fri, 10 Apr 2009 14:16:55 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Krzysztof Helt <krzysztof.h1@...zta.fm>
Cc:	righi.andrea@...il.com, eric.y.miao@...il.com,
	linux-kernel@...r.kernel.org, geert@...ux-m68k.org, rjw@...k.pl,
	arvidjaar@...il.com, adaplas@...il.com,
	linux-fbdev-devel@...ts.sourceforge.net,
	linux-pm@...ts.linux-foundation.org, davej@...hat.com,
	harvey.harrison@...il.com, hannes@...xchg.org,
	stefanr@...6.in-berlin.de
Subject: Re: [REGRESSION] commit 66c1ca0: {fbmem: fix fb_info->lock and
 mm->mmap_sem ...} causes Xfbdev not working

On Fri, 10 Apr 2009 22:21:22 +0200
Krzysztof Helt <krzysztof.h1@...zta.fm> wrote:

> On Thu, 9 Apr 2009 14:58:51 +0200
> Andrea Righi <righi.andrea@...il.com> wrote:
> 
> > On Thu, Apr 09, 2009 at 07:36:24PM +0800, Eric Miao wrote:
> > > This happens on my Marvell PXA310-based Littleton platform with
> > > Angstrom Distribution. The offending paths are many:
> > > 
> > > FBIOPUT_VSCREENINFO:
> > >   lock_fb_info()
> > >   --> fb_set_var()
> > >     --> fb_notifier_call_chain() [FBINFO_MISC_USEREVENT]
> > >       --> fbcon_event_notifier() [FB_EVENT_MODE_CHANGE]
> > >         --> lock_fb_info()
> > > 
> > > OK, now hang. I'd suggest a clean fix to the original assumption of
> > > circular locking
> > > issue and revert this commit first.
> > > 
> > > -- 
> > > Cheers
> > > - eric
> > 
> > I can agree to revert 66c1ca019078220dc1bf968f2bb18421100ef147, since I
> > don't have a clean fix for this. Pushing down fb_info->lock in
> > fb_set_var() excluding to call fb_notifier_call_chain with fb_info->lock
> > held doesn't seem to be so trivial...
> > 
> > However, reverting this will re-introduce the circular locking
> > dependency fb_info->lock => mm->mmap_sem => fb_info->lock.
> > 
> 
> If anybody is interested I have looked into the code of the fb_mmap()
> and have found that the problem Andrea tried to fix is caused
> only by drivers which implements their own fb_mmap().
> 
> The correct solution is to push the info->lock mutex into
> the fb_mmap() implemented inside the drivers. Some drivers
> may not need it (the ones which only calculate offsets
> and do not "real" iommaping).
> 
> I'll try to prepare a patch after the Easters.

Thanks.

> This will require a revert
>  of the 66c1ca commit.

yes, I'll be queueing a revert of that when I get that far through my
backlog.  Today or tomorrow.

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