Re: [PATCH][RFC] iio: core: add a class hierarchy on iio device lock

[Jonathan Cameron <jic23@kernel.org>]

On Tue, 15 Oct 2019 23:11:43 +0200
Lars-Peter Clausen <lars@metafoo.de> wrote:

> On 10/14/19 5:59 PM, Olivier MOYSAN wrote:
> > Hello Jonathan,
> > 
> > Thanks for your comment.
> > 
> > On 10/12/19 10:57 AM, Jonathan Cameron wrote:  
> >> On Fri, 11 Oct 2019 17:13:14 +0200
> >> Olivier Moysan <olivier.moysan@st.com> wrote:
> >>  
> >>> The aim of this patch is to correct a recursive locking warning,
> >>> detected when setting CONFIG_PROVE_LOCKING flag (as shown in message below).
> >>> This message was initially triggered by the following call sequence
> >>> in stm32-dfsdm-adc.c driver, when using IIO hardware consumer interface.
> >>>
> >>> in stm32_dfsdm_read_raw()
> >>> 	iio_device_claim_direct_mode
> >>> 		mutex_lock(&indio_dev->mlock);			-> lock on dfsdm device
> >>> 	iio_hw_consumer_enable
> >>> 		iio_update_buffers
> >>> 			mutex_lock(&indio_dev->mlock);		-> lock on hw consumer device  
> >> Hmm.  I'm not sure I follow the logic.  That lock is
> >> for one thing and one thing only, preventing access
> >> to the iio device that are unsafe when it is running
> >> in a buffered mode.  We shouldn't be in a position where
> >> we both say don't do this if we are in buffered mode, + enter
> >> buffered mode whilst doing this, or we need special functions
> >> for entering buffering mode if in this state.  We are in
> >> some sense combining internal driver logic with overall
> >> IIO states.  IIO shouldn't care that the device is using
> >> the same methods under the hood for buffered and non
> >> buffered operations.
> >>
> >> I can't really recall how this driver works.   Is it actually
> >> possible to have multiple hw_consumers at the same time?
> >>
> >> So do we end up with multiple buffers registered and have
> >> to demux out to the read_raw + the actual buffered path?
> >> Given we have a bit of code saying grab one sample, I'm
> >> going to guess we don't...
> >>
> >> If so, the vast majority of the buffer setup code in IIO
> >> is irrelevant here and we just need to call a few of
> >> the callbacks from this driver directly... (I think
> >> though I haven't chased through every corner.
> >>
> >> I'd rather avoid introducing this nesting for a corner
> >> case that makes no 'semantic' sense in IIO as it leaves us
> >> in two separate states at the same time that the driver
> >> is trying to make mutually exclusive.  We can't both
> >> not be in buffered mode, and in buffered mode.
> >>
> >> Thanks and good luck with this nasty corner!
> >>
> >> Jonathan
> >>  
> > Here I consider the following use case:
> > A single conversion is performed. The dfsdm (filter) is chained with a 
> > front-end, which can be an ADC or a sensor. So we have two IIO devices, 
> > the dfsdm and its front-end handled through the hw consumer interface.
> > 
> > You are right. There is something wrong here, in buffered/non-buffered 
> > mode mixing.
> > iio_hw_consumer_enable() call is used to enable the front-end device. 
> > But this interface is intended for buffered mode.
> > So this is not coherent with the expected single conversion mode, 
> > indeed. Another interface is required to manage the front-end device. I 
> > have a poor knowledge of iio framework, but it seems to me that there is 
> > no interface to manage this.
> > 
> > My understanding regarding mlock, is that it is used to protect the 
> > state of the iio device.
> > I we want to do a conversion from the chained devices, I think we need 
> > to activate the first device
> > and keep it performing conversion, as long as the second device has done 
> > its conversion.
> > We need to protect both devices, and we should have to do it in a nested 
> > way.
> > So, I guess that anyway, nested mutexes would be required in this case.
> >  
> 
> Others like regmap have solved this by having a lockclass per instance.
> Although that is not ideal either since it will slow down lockdep.
> 
> See
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/regmap.h#n629

It'll take me a while to get back to this as my understanding is
currently very limited.  Poke me if I've not replied in a few weeks.

Thanks,

Jonathan