linux-kernel - Re: [PATCH] mm: memory_hotplug: use put_device() if device

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Message-ID: <20180502142741.GI26305@dhcp22.suse.cz>
Date:   Wed, 2 May 2018 16:27:41 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     arvindY <arvind.yadav.cs@...il.com>
Cc:     gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: memory_hotplug: use put_device() if device_register
 fail

On Sat 28-04-18 11:05:44, arvindY wrote:
> 
> 
> On Friday 27 April 2018 08:26 PM, Michal Hocko wrote:
> > On Thu 26-04-18 21:12:09, Arvind Yadav wrote:
> > > if device_register() returned an error. Always use put_device()
> > > to give up the initialized reference and release allocated memory.
> > Is this patch correct? The docummentation says
> >   * NOTE: _Never_ directly free @dev after calling this function, even
> >   * if it returned an error! Always use put_device() to give up your
> >   * reference instead.
> > 
> > but we do not have _our_ reference in this path AFAICS. Maybe this is
> > just a documentation issue? How have you tested this change btw.?
> The document is correct. Here device_register() will initialize object by
> making reference count as 1 and also increment reference count for device.
> 
> device_register() {
>    device_initialize()->kobject_init()->kref_init() - initialize object(
> reference count = 1).
>    device_add()->get_device() - increment reference count for device.
> }
> 
> If device_register() will fail then we have to release the object by making
> reference count 0.

I am confused. If device_register intializes the reference to 1 then why
it doesn't decrement it on a failure path and rather expects the caller
to do that? The doc as I read it means that we should only drop a
reference if we have our own one before calling device_register.

Or what do I miss here?
-- 
Michal Hocko
SUSE Labs