linux-kernel - Re: [PATCH 0/2] use DEFINE_DEBUGFS_ATTRIBUTE instead of DEFINE_SIMPLE

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Date:   Sun, 29 Mar 2020 15:46:17 +0200
From:   Lars-Peter Clausen <lars@...afoo.de>
To:     Rohit Sarkar <rohitsarkar5398@...il.com>,
        Jonathan Cameron <jic23@...nel.org>
Cc:     linux-iio@...r.kernel.org, dragos.bogdan@...log.com,
        Michael Hennerich <Michael.Hennerich@...log.com>,
        Stefan Popa <stefan.popa@...log.com>,
        Hartmut Knaack <knaack.h@....de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        linux-kernel@...r.kernel.org,
        "Ardelean, Alexandru" <alexandru.Ardelean@...log.com>,
        zhong jiang <zhongjiang@...wei.com>
Subject: Re: [PATCH 0/2] use DEFINE_DEBUGFS_ATTRIBUTE instead of
 DEFINE_SIMPLE_ATTRIBUTE

On 3/29/20 1:34 PM, Rohit Sarkar wrote:
> On Sun, Mar 29, 2020 at 10:38:18AM +0100, Jonathan Cameron wrote:
>> On Sat, 28 Mar 2020 12:04:53 +0530
>> Rohit Sarkar <rohitsarkar5398@...il.com> wrote:
>>
>>> The debugfs_create_file_unsafe method does not protect the fops given to
>>> it from file removal. It must be used with DEFINE_DEBUGFS_ATTRIBUTE
>>> which makes the fops aware of the file lifetime.
>>>
>>> Further using DEFINE_DEBUGFS_ATTRIBUTE along with
>>> debugfs_create_file_unsafe significantly reduces the overhead introduced by
>>> debugfs_create_file which creates a lifetime managing proxy around each
>>> fops handed in. Refer [1] for more on this.
>>>
>>> Fixes the following warnings reported by coccinelle:
>>> drivers/iio/imu//adis16460.c:126:0-23: WARNING: adis16460_flash_count_fops should be defined with DEFINE_DEBUGFS_ATTRIBUTE
>>> drivers/iio/imu//adis16460.c:108:0-23: WARNING: adis16460_product_id_fops should be defined with DEFINE_DEBUGFS_ATTRIBUTE
>>> drivers/iio/imu//adis16460.c:90:0-23: WARNING: adis16460_serial_number_fops should be defined with DEFINE_DEBUGFS_ATTRIBUTE
>>> drivers/iio/imu//adis16400.c:278:0-23: WARNING: adis16400_flash_count_fops should be defined with DEFINE_DEBUGFS_ATTRIBUTE
>>> drivers/iio/imu//adis16400.c:261:0-23: WARNING: adis16400_product_id_fops should be defined with DEFINE_DEBUGFS_ATTRIBUTE
>>>
>>> [1]: https://lists.gt.net/linux/kernel/2369498
>>>
>>> Rohit Sarkar (2):
>>>    iio: imu: adis16400: use DEFINE_DEBUGFS_ATTRIBUTE instead of
>>>      DEFINE_SIMPLE_ATTRIBUTE
>>>    iio: imu: adis16460: use DEFINE_DEBUGFS_ATTRIBUTE instead of
>>>      DEFINE_SIMPLE_ATTRIBUTE
>>>
>>>   drivers/iio/imu/adis16400.c | 4 ++--
>>>   drivers/iio/imu/adis16460.c | 6 +++---
>>>   2 files changed, 5 insertions(+), 5 deletions(-)
>>>
>> Hi Rohit,
> Hey,
>> You've opened a can of worms with this one.  There as a previous series
>> posted doing exactly this change back in 2019 by Zhong Jiang (cc'd)
>>
>> At the time I did a bit of looking into why this had been universally taken
>> up cross tree and turned out there are some potential issues.
>>
>> Alexandru added it to the list of things to test, but I guess it got
>> buried under other work and is still outstanding.
>>
>> https://lkml.org/lkml/2019/10/30/144
> Acc. to the patch by Zhong this change kind of comes off as a cosmetic
> change as in the commit message he mentions "it is more clear".
>
> But there is certainly more to it than that:
> In the current scenario since we are using debugfs_create_file_unsafe
> the file has no protection whatsoever against removal.

The drivers you are patching all use debugfs_create_file() as far as I 
can see.

The way I understand it using DEFINE_DEBUGFS_ATTRIBUTE without switching 
to debugfs_create_file_unsafe() will not make a difference. There will 
only be more overhead since the files are protected twice.

- Lars