Re: [PATCH 0/2] use DEFINE_DEBUGFS_ATTRIBUTE instead of DEFINE_SIMPLE_ATTRIBUTE

[Lars-Peter Clausen]

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@xxxxxxxxx> 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