linux-kernel - Re: [KJ][PATCH] Correct misc_register return code handling in several drivers

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Message-ID: <20061025131747.GA8141@hmsreliant.homelinux.net>
Date:	Wed, 25 Oct 2006 09:17:47 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	kernel-janitors@...ts.osdl.org, kjhall@...ibm.com, akpm@...l.org,
	maxk@...lcomm.com, linux-kernel@...r.kernel.org
Subject: Re: [KJ][PATCH] Correct misc_register return code handling in several drivers

On Wed, Oct 25, 2006 at 08:42:42AM +1000, Benjamin Herrenschmidt wrote:
> On Tue, 2006-10-24 at 08:53 -0400, Neil Horman wrote:
> > On Tue, Oct 24, 2006 at 01:34:34PM +1000, Benjamin Herrenschmidt wrote:
> > > On Mon, 2006-10-23 at 13:19 -0400, Neil Horman wrote:
> > > > Hey All-
> > > > 	Janitor patch to clean up return code handling and exit from failed
> > > > calls to misc_register accross several modules.
> > > 
> > > The patch doesn't match the description... What are those INIT_LIST_HEAD
> > > things ? Is this something I've missed or is this a new requirement for
> > > all misc devices ? Can't it be statically initialized instead ?
> > > 
> > 
> > The INIT_LIST_HEAD is there to prevent a potential oops on module removal.
> > misc_register, if it fails, leaves miscdevice.list unchanged.  That means its
> > next and prev pointers contain NULL or garbage, when both pointers should contain
> > &miscdevice.list. If we don't do that, then there is a chance we will oops on
> > module removal when we do a list_del in misc_deregister on the moudule_exit
> > routine.  I could have done this statically, but I thought it looked cleaner to
> > do it with the macro in the code.
> 
> Hrm... I see, but I still for some reason don't like it that much.. I'd
> rather have misc_register() do the initialisation unconditionally before
> it can fail, don't you think ?
> 
> We would theorically have a similar problem with any driver that does
> 
> 
> xxxx_register(&static_struct)
> 
> and
> 
> xxxx_unregister(&static_struct)
> 
> (pci, usb, etc...)
> 
> As long as there are list heads involved. I think the proper solution
> here is to have either the unregister be smart and test for NULL/NULL or
> the register initialize those fields before it has a chance to fail.
> 
> Ben.
> 

I agreed with you in my last note regarding this, I think moving the
INIT_LIST_HEAD inside the misc_register function is a good idea, but since this
is a cleanup patch with several other fixups in it, I'd just as soon get this
integrated, and make that change in a separate patch.

Regards
Neil

-- 
/***************************************************
 *Neil Horman
 *Software Engineer
 *gpg keyid: 1024D / 0x92A74FA1 - http://pgp.mit.edu
 ***************************************************/
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