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

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Message-ID: <20090409125850.GA2727@linux>
Date:	Thu, 9 Apr 2009 14:58:51 +0200
From:	Andrea Righi <righi.andrea@...il.com>
To:	Eric Miao <eric.y.miao@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Krzysztof Helt <krzysztof.h1@...zta.fm>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Andrey Borzenkov <arvidjaar@...il.com>,
	"Antonino A. Daplas" <adaplas@...il.com>,
	linux-fbdev-devel@...ts.sourceforge.net,
	linux-pm@...ts.linux-foundation.org, Dave Jones <davej@...hat.com>,
	Harvey Harrison <harvey.harrison@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Stefan Richter <stefanr@...6.in-berlin.de>
Subject: Re: [REGRESSION] commit 66c1ca0: {fbmem: fix fb_info->lock and
	mm->mmap_sem ...} causes Xfbdev not working

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.

-Andrea
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