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

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Date:	Sat, 11 Apr 2009 08:04:15 +0200
From:	Krzysztof Helt <krzysztof.h1@...zta.fm>
To:	Andrea Righi <righi.andrea@...il.com>
Cc:	Eric Miao <eric.y.miao@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	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 Sat, 11 Apr 2009 00:05:22 +0200
Andrea Righi <righi.andrea@...il.com> wrote:

> 
> mmmh... I may have missed something, but the common fb_mmap() should
> acquire mm->mmap_sem and then info->lock, while fb_ioctl() can do that
> in reverse order (info->lock first and then mm->mmap_sem) causing the
> circular locking dependency. Are you sure that pushing info->lock down
> each driver's fb_mmap will fix the problem?

Right. The fb_mmap is called with the mmap_sem already held.
I will try other possibilities like breaking info->lock() into two
mutexex.

Something should be done to solve this problem.

Best regards,
Krzysztof

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