linux-kernel - Re: [PATCH] nvmem: fix unregistering device in nvmem

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Date:   Wed, 22 Dec 2021 10:02:43 +0100
From:   Rafał Miłecki <zajec5@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Johan Hovold <johan@...nel.org>
Cc:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        Andrey Smirnov <andrew.smirnov@...il.com>,
        linux-kernel@...r.kernel.org,
        Rafał Miłecki <rafal@...ecki.pl>
Subject: Re: [PATCH] nvmem: fix unregistering device in nvmem_register() error
 path

On 22.12.2021 09:56, Greg Kroah-Hartman wrote:
> On Wed, Dec 22, 2021 at 09:38:27AM +0100, Johan Hovold wrote:
>> On Wed, Dec 22, 2021 at 08:44:44AM +0100, Greg Kroah-Hartman wrote:
>>> On Tue, Dec 21, 2021 at 06:46:01PM +0100, Rafał Miłecki wrote:
>>>> On 21.12.2021 17:06, Greg Kroah-Hartman wrote:
>>>>> On Tue, Dec 21, 2021 at 04:45:50PM +0100, Rafał Miłecki wrote:
>>>>>> From: Rafał Miłecki <rafal@...ecki.pl>
>>>>>>
>>>>>> 1. Drop incorrect put_device() calls
>>>>>>
>>>>>> If device_register() fails then underlaying device_add() takes care of
>>>>>> calling put_device() if needed. There is no need to do that in a driver.
>>>>>
>>>>> Did you read the documentation for device_register() that says:
>>>>>
>>>>>    * 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.
>>>>
>>>> I clearly tried to be too smart and ignored documentation.
>>>>
>>>> I'd say device_add() behaviour is rather uncommon and a bit unintuitive.
>>>> Most kernel functions are safe to assume to do nothing that requires
>>>> cleanup if they fail.
>>>>
>>>> E.g. if I call platform_device_register() and it fails I don't need to
>>>> call anything like platform_device_put(). I just free previously
>>>> allocated memory.
>>>
>>> And that is wrong.
>>
>> It seems Rafał is mistaken here too; you certainly need to call
>> platform_device_put() if platform_device_register() fail, even if many
>> current users do appear to get this wrong.
> 
> A short search found almost everyone getting this wrong.  Arguably
> platform_device_register() can clean up properly on its own if we want
> it to do so.  Will take a lot of auditing of the current codebase first
> to see if it's safe...

If so many people get it wrong maybe that is indded an unintuitive
design?

I'll better hide now ;)