linux-kernel - Re: [PATCH] net: hns: use put_device() if device

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Message-ID: <CAFLxGvw1U2OZn4kU1O6uJ17yWYf1deE=+Cr56Foffdq7Q=5u3A@mail.gmail.com>
Date:   Mon, 12 Mar 2018 18:29:32 +0100
From:   Richard Weinberger <richard.weinberger@...il.com>
To:     arvindY <arvind.yadav.cs@...il.com>
Cc:     David Miller <davem@...emloft.net>, yisen.zhuang@...wei.com,
        salil.mehta@...wei.com, linyunsheng@...wei.com,
        LKML <linux-kernel@...r.kernel.org>, netdev@...r.kernel.org,
        linux-mtd@...ts.infradead.org
Subject: Re: [PATCH] net: hns: use put_device() if device_register fail

On Mon, Mar 12, 2018 at 5:27 PM, arvindY <arvind.yadav.cs@...il.com> wrote:
>
>
> On Monday 12 March 2018 08:13 PM, David Miller wrote:
>>
>> From: Arvind Yadav <arvind.yadav.cs@...il.com>
>> Date: Fri,  9 Mar 2018 16:11:17 +0530
>>
>>> if device_register() returned an error! Always use put_device()
>>> to give up the reference initialized.
>>>
>>> Signed-off-by: Arvind Yadav <arvind.yadav.cs@...il.com>
>>
>> I do not see anything giving cls_dev an initial non-zero reference
>> count before this device_register() call.
>
> Yes,  you are correct there is nothing to release (hnae_release).

Wait, this is not what DaveM said.
Since you sent also patches to MTD I care about this too.

Do we have to call put_device() in any case after a failure of
device_register() or not?
In this case the release function is empty, so nothing is going to released.
But technically a put_device() is needed and correct according to your
change log.

I have to admit I don't know all details of the driver core, maybe you
can help me.
device_register() calls device_initialize() followed by device_add().
device_initialize() does a kobject_init() which again sets the
reference counter to 1 via kref_init().
In the next step device_add() does a get_device() --> reference
counter goes up to 2.
But in the exit path of the function a put_device() is done, also in
case of an error.
So device_register() always returns with reference count 1.

If I understand this correctly a put_device() is mandatory.
Also in this driver.
Can you please enlighten me? :-)

-- 
Thanks,
//richard