linux-kernel - Re: Deadlock in fb and tty

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Message-ID: <52308D47.409@hurleysoftware.com>
Date:	Wed, 11 Sep 2013 11:33:27 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	John Tapsell <johnflux@...il.com>
CC:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Deadlock in fb and tty

On 09/11/2013 05:25 AM, John Tapsell wrote:
> Hi,
>    I'm consistently and constantly hitting a deadlock.
>
> console_callback in drivers/tty/vt/vt.c does:  console_lock()  and then calls:
>    do_blank_screen, which calls:
>      vc->vc_sw->con_blank(..)  which can be a pointer to the function:
>        fbcon_blank in video/console/fbcon.c.  This is missing a
> WARN_CONSOLE_UNLOCKED.  This calls:
>          fbcon_generic_blank which does lock_fb_info(info)
>
> So we have a console_lock() followed by a lock_fb_info(info).
>
> Now if while that is running, we have an ioctl call:
>
> do_fb_ioctl in drivers/video/fbmem.c which does:
>                 if (!lock_fb_info(info))
>                          return -ENODEV;
>                  console_lock();
>
>
> So it tries to acquire the same locks in the reverse order.  This
> deadlocks consistently for me.
>
> (I'm also curious why I'm hitting this continually, when everyone else
> seems to be okay.)
>
> I understand that this is really difficult to fix, but if anyone has
> even a suggestion on how to hack it to make it work for me, I'd be
> very grateful.

As a temporary workaround, you can disable timed console blanking on the
kernel command line with 'consoleblank=0'.

Regards,
Peter Hurley

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