linux-kernel - Re: [PATCH] fbcon: fix deadlock in fbcon_generic

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Date:	Fri, 27 Sep 2013 10:15:48 +0300
From:	Tomi Valkeinen <tomi.valkeinen@...com>
To:	John Tapsell <johnflux@...il.com>
CC:	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Dave Airlie <airlied@...hat.com>,
	<linux-fbdev@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fbcon: fix deadlock in fbcon_generic_blank()

Hi,

On 18/09/13 01:29, John Tapsell wrote:
> Do not lock fb_info when calling sending the FB_EVENT_CONBLANK
> event.
> 
> In fbmem.c, the semantics are that we acquire the lock_fb_info first,
> and then console_lock.  However when fbcon.c fbcon_generic_blank() is
> called, the console lock could already be held.  Locking fb_info can
> thus cause a deadlock.

So has this happened for you? Or is it just theoretical?

> fbmem.c sends the FB_EVENT_BLANK without locking lock_fb_info first, so
> this change introduces similar behaviour.

I don't think this is true. FB_EVENT_BLANK is sent in fb_blank(). That
one is called when FBIOBLANK ioctl is called, and it does lock_fb_info().

I'm not familiar with the console code, but removing a lock makes me
feel rather uneasy... But looking at the code, I can also see that
console_lock could already be held, so something here definitely looks
broken.

The only place using FB_EVENT_CONBLANK seems to be backlight, and if I'm
not mistaken, it has its own lock, and doesn't depend on the fb_info
being locked.

 Tomi

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