Re: [PATCH] x86: mcheck: call put_device on device_register failure

["Chen, Gong" <gong.chen@linux.intel.com>]

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On Wed, Dec 04, 2013 at 07:39:07PM +0100, Levente Kurusa wrote:
> Date:	Wed, 04 Dec 2013 19:39:07 +0100
> From: Levente Kurusa <levex@linux.com>
> To: Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@kernel.org>, Thomas
>  Gleixner <tglx@linutronix.de>, Tony Luck <tony.luck@intel.com>, "H. Peter
>  Anvin" <hpa@zytor.com>, x86@kernel.org, EDAC <linux-edac@vger.kernel.org>,
>  LKML <linux-kernel@vger.kernel.org>
> Subject: Re: [PATCH] x86: mcheck: call put_device on device_register failure
> User-Agent: Mozilla/5.0 (X11; Linux i686; rv:24.0) Gecko/20100101
>  Thunderbird/24.1.0
> 
> 2013-12-04 08:38, Chen, Gong:
> > On Tue, Dec 03, 2013 at 06:01:50PM +0100, Borislav Petkov wrote:
> >> Date: Tue, 3 Dec 2013 18:01:50 +0100
> >> From: Borislav Petkov <bp@alien8.de>
> >> To: "Chen, Gong" <gong.chen@linux.intel.com>
> >> Cc: Levente Kurusa <levex@linux.com>, Ingo Molnar <mingo@kernel.org>,
> >>  Thomas Gleixner <tglx@linutronix.de>, Tony Luck <tony.luck@intel.com>, "H.
> >>  Peter Anvin" <hpa@zytor.com>, x86@kernel.org, EDAC
> >>  <linux-edac@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>
> >> Subject: Re: [PATCH] x86: mcheck: call put_device on device_register failure
> >> User-Agent: Mutt/1.5.21 (2010-09-15)
> >>
> >> Can you please fix your
> >>
> >> Mail-Followup-To:
> >>
> >> header? It is impossible to reply to your emails without fiddling with
> >> the To: and Cc: by hand which gets very annoying over time.
> > 
> > I add some configs in my muttrc. Hope it works.
> > 
> >>
> >> On Mon, Dec 02, 2013 at 09:23:30PM -0500, Chen, Gong wrote:
> >>> I have some concerns about it. if device_register is failed, it will
> >>> backtraces all kinds of conditions automatically, including put_device
> >>> definately. So do we really need an extra put_device when it returns
> >>> failure?
> >>
> >> Do you mean the "done:" label in device_add() which does put_device()
> >> and which gets called by device_register()?
> >>
> > 
> > Not only. I noticed that another put_device under label "Error:".
> > 
> 
> That label is called when we failed to add the kobject to its parent.
> It just puts the parent of the device. I don't think it has anything
> to do with us put_device()-ing the actual device too.
> 
OK, you are right. I read some kobject related codes and get:

static inline void kref_init(struct kref *kref)
{
        atomic_set(&kref->refcount, 1);
}

The init refcount is 1, which means even if we meet an error and put_device
in device_add, we still need an extra put_device to make refcount = 0
and then release the dev object.

BTW, from the comments of device_register:

"NOTE: _Never_ directly free @dev after calling this function, even
 if it returned an error! Always use put_device() to give up the
 reference initialized in this function instead. "

Many caller don't follow this logic. For example:
in arch/arm/common/locomo.c
locomo_init_one_child
...
        ret = device_register(&dev->dev);
        if (ret) {
out:
                kfree(dev);
        }
...
 
in arch/parisc/kernel/drivers.c
create_tree_node
...
        if (device_register(&dev->dev)) {
                kfree(dev);
                return NULL;
        }
...

etc.

Maybe we need one more patch to fix them all. :-)
> -- 
> Regards,
> Levente Kurusa
> --
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