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

[Rohit Sarkar <rohitsarkar5398@gmail.com>]

On Tue, Mar 31, 2020 at 01:08:00PM +0200, Lars-Peter Clausen wrote:
> On 3/31/20 12:58 PM, Sa, Nuno wrote:
> > Hi Rohit,
> > 
> > > From: linux-iio-owner@vger.kernel.org <linux-iio-owner@vger.kernel.org> On
> > > Behalf Of Ardelean, Alexandru
> > > Sent: Montag, 30. März 2020 11:20
> > > To: jic23@kernel.org; rohitsarkar5398@gmail.com
> > > Cc: stefan.popa@analog.com; zhongjiang@huawei.com; linux-
> > > iio@vger.kernel.org; Bogdan, Dragos <Dragos.Bogdan@analog.com>;
> > > pmeerw@pmeerw.net; knaack.h@gmx.de; Hennerich, Michael
> > > <Michael.Hennerich@analog.com>; linux-kernel@vger.kernel.org;
> > > lars@metafoo.de
> > > Subject: Re: [PATCH 0/2] use DEFINE_DEBUGFS_ATTRIBUTE instead of
> > > DEFINE_SIMPLE_ATTRIBUTE
> > > 
> > > On Sun, 2020-03-29 at 10:38 +0100, Jonathan Cameron wrote:
> > > > On Sat, 28 Mar 2020 12:04:53 +0530
> > > > Rohit Sarkar <rohitsarkar5398@gmail.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,
> > > > 
> > > > 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.
> > > > 
> > > yep
> > > my bad;
> > > will try to make room these days for that old one
> > > 
> > > 
> > I don't have the exact parts that this patch is touching but I have other parts where this patch
> > applies and should be same. So, the idea to test this is to read the files in debugfs? Maybe also
> > some unbind + binding?
> > 
> > I will try to test this still today...
> 
> The stress test is to open the debugfs file, then unbind the device and then
> read from the still open debugfs file.
Yes, also just to be sure, we need to test DEFINE_DEBUGFS_ATTRIBUTE
along with debugfs_create_file_unsafe. I will send out another patch
that changes debugfs_create_file to debugfs_create_file_unsafe and then
that can be tested.

> - Lars

Thanks,
Rohit