From: Arnd Bergmann <arnd@arndb.de>
To: Jiri Slaby <jslaby@suse.cz>
Subject: Re: [PATCH 01/15] Char: lp, protect LPGETSTATUS with port_mutex
Date: Tue, 7 May 2013 16:48:34 +0200
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Cc: jirislaby@gmail.com, linux-kernel@vger.kernel.org,
        "salina@us.ibm.com" <salina@us.ibm.com>, okir@suse.de,
        "Greg Kroah-Hartman" <gregkh@linuxfoundation.org>
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On Tuesday 07 May 2013, Jiri Slaby wrote:
> From: "salina@us.ibm.com" <salina@us.ibm.com>
> 
> The patch fixes a problem in the lp driver that can cause oopses as
> follows:
> process A:      calls lp_write, which in turn calls
>                 parport_ieee1284_write_compat, and that invokes
>                 parport_wait_peripheral
> process B:      meanwhile does an ioctl(LPGETSTATUS), which call
>                 lp_release_parport when done. This function will set
>                 physport->cad = NULL.
> process A:      parport_wait_peripheral tries to dereference
>                 physport->cad and dies
> 
> So, protect that code with the port_mutex in order to protect against
> simultaneous calls to lp_read/lp_write.
> 
> Similar protection is probably required for ioctl(LPRESET)...
> 
> This patch was done by IBM a while back and we (at suse) have that
> since at least 2004 in our repos. Let's make it upstream.

Hmm,  it seems the driver has changed a bit since 2004, at least when
I added the lp_mutex to lp_open()/lp_ioctl(). It's probably worth
taking a look at the bigger picture now, to combine lp_mutex with
lp_table[minor].port_mutex. I don't see any reason why we can't always
use the per-device mutex. The only shared variable is the lp_count 
number, and that is not protected under lp_mutex today, and presumably
not updated at run time either.

	Arnd
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