Re: [PATCH] iio: adc: exynos_adc: Replace indio_dev->mlock with own device lock

[Jonathan Cameron <jic23@kernel.org>]

On Thu, 27 Aug 2020 11:53:44 +0300
Alexandru Ardelean <ardeleanalex@gmail.com> wrote:

> On Thu, Aug 27, 2020 at 9:57 AM Krzysztof Kozlowski <krzk@kernel.org> wrote:
> >
> > On Wed, Aug 26, 2020 at 04:22:03PM +0300, Alexandru Ardelean wrote:  
> > > From: Sergiu Cuciurean <sergiu.cuciurean@analog.com>
> > >
> > > As part of the general cleanup of indio_dev->mlock, this change replaces
> > > it with a local lock, to protect potential concurrent access to the
> > > completion callback during a conversion.  
> >
> > I don't know the bigger picture (and no links here for general cleanup)
> > but I assume it is part of wider work and that mlock is unwanted. In
> > such case:
> >
> > Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
> >
> > If it is part of some bigger work, please put a link to lore.kernel.org
> > under separators ---, so everyone can get the context.  
> 
> Will keep that in mind.
> I am not sure if there is a lore.kernel.org link that's easy to find
> for a discussion on this topic, maybe I can describe it here and use
> the link [from this later].
> 
> This was something that popped up during reviews we got from Jonathan
> [or others], saying "please don't use indio_dev->mlock, that is an IIO
> framework lock, and an IIO driver should not use it".

Shortest one is the docs for that lock say don't use it directly in
a driver :)

https://elixir.bootlin.com/linux/latest/source/include/linux/iio/iio.h#L495

> Reasons include [and some may be repeated a bit]:
> - this could cause a deadlock if the IIO framework holds this lock and
> an IIO driver also tries to get a hold of this lock
> - similar to the previous point, this mlock is taken by
> iio_device_claim_direct_mode() and released by
> iio_device_release_direct_mode() ; which means that mlock aims to
> become more of an IIO framework lock, than a general usage lock;
> - this wasn't policed/reviewed intensely in the older driver [a few
> years ago], but has become a point in recent reviews;
> - if we want to develop/enhance the IIO framework, some elements like
> this need to be taken care of, as more drivers get added and more
> complexity gets added;

One side note here is we want to make all this [INTERN] state in 
struct iio_dev opaque to drivers.  It'll take a while as the boundary
gets crossed in various drivers.

> - there is an element of fairness [obviously], where someone writing a
> new IIO driver, takes an older one as example, and gets hit on the
> review; the person feels they did a good job in mimicking the old
> driver; their feeling is correct; the IIO framework should provide
> good references and/or cleanup existing drivers;
> - same as the previous point, we don't want to keep telling people
> writing new IIO drivers [and starting out with IIO] to "not use mlock
> [because it was copied from an old driver]"; it's more/needless review
> work

Good explanation.

Thanks,

Jonathan

> 
> 
> >
> >
> > Best regards,
> > Krzysztof
> >  
> > >
> > > Signed-off-by: Sergiu Cuciurean <sergiu.cuciurean@analog.com>
> > > Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
> > > ---
> > >  drivers/iio/adc/exynos_adc.c | 12 ++++++++----
> > >  1 file changed, 8 insertions(+), 4 deletions(-)
> > >