Re: [PATCH 13/15] iio: health: max30100: do not use internal iio_dev lock

["Nuno Sá" <noname.nuno@gmail.com>]

On Sun, 2022-10-02 at 12:03 +0100, Jonathan Cameron wrote:
> On Mon, 26 Sep 2022 12:06:13 +0200
> Nuno Sá <noname.nuno@gmail.com> wrote:
> 
> > On Sat, 2022-09-24 at 16:53 +0100, Jonathan Cameron wrote:
> > > On Tue, 20 Sep 2022 17:10:33 +0200
> > > Miquel Raynal <miquel.raynal@bootlin.com> wrote:
> > >   
> > > > Hi Nuno,
> > > > 
> > > > noname.nuno@gmail.com wrote on Tue, 20 Sep 2022 16:56:01 +0200:
> > > >   
> > > > > On Tue, 2022-09-20 at 15:53 +0200, Miquel Raynal wrote:    
> > > > > > Hi Nuno,
> > > > > > 
> > > > > > Nuno.Sa@analog.com wrote on Tue, 20 Sep 2022 13:15:32
> > > > > > +0000:
> > > > > >       
> > > > > > > > -----Original Message-----
> > > > > > > > From: Miquel Raynal <miquel.raynal@bootlin.com>
> > > > > > > > Sent: Tuesday, September 20, 2022 2:56 PM
> > > > > > > > To: Sa, Nuno <Nuno.Sa@analog.com>
> > > > > > > > Cc: linux-arm-kernel@lists.infradead.org;
> > > > > > > > linux-rockchip@lists.infradead.org;
> > > > > > > > linux-amlogic@lists.infradead.org; linux-imx@nxp.com;
> > > > > > > > linux-
> > > > > > > > iio@vger.kernel.org; Chunyan Zhang
> > > > > > > > <zhang.lyra@gmail.com>;
> > > > > > > > Hennerich,
> > > > > > > > Michael <Michael.Hennerich@analog.com>; Martin
> > > > > > > > Blumenstingl
> > > > > > > > <martin.blumenstingl@googlemail.com>; Sascha Hauer
> > > > > > > > <s.hauer@pengutronix.de>; Cixi Geng
> > > > > > > > <cixi.geng1@unisoc.com>;
> > > > > > > > Kevin
> > > > > > > > Hilman <khilman@baylibre.com>; Vladimir Zapolskiy
> > > > > > > > <vz@mleia.com>;
> > > > > > > > Pengutronix Kernel Team <kernel@pengutronix.de>;
> > > > > > > > Alexandru
> > > > > > > > Ardelean
> > > > > > > > <aardelean@deviqon.com>; Fabio Estevam
> > > > > > > > <festevam@gmail.com>;
> > > > > > > > Andriy
> > > > > > > > Tryshnivskyy <andriy.tryshnivskyy@opensynergy.com>;
> > > > > > > > Haibo
> > > > > > > > Chen
> > > > > > > > <haibo.chen@nxp.com>; Shawn Guo <shawnguo@kernel.org>;
> > > > > > > > Hans
> > > > > > > > de
> > > > > > > > Goede <hdegoede@redhat.com>; Jerome Brunet
> > > > > > > > <jbrunet@baylibre.com>;
> > > > > > > > Heiko Stuebner <heiko@sntech.de>; Florian Boor
> > > > > > > > <florian.boor@kernelconcepts.de>; Regus, Ciprian
> > > > > > > > <Ciprian.Regus@analog.com>; Lars-Peter Clausen
> > > > > > > > <lars@metafoo.de>;
> > > > > > > > Andy
> > > > > > > > Shevchenko <andy.shevchenko@gmail.com>; Jonathan
> > > > > > > > Cameron
> > > > > > > > <jic23@kernel.org>; Neil Armstrong
> > > > > > > > <narmstrong@baylibre.com>;
> > > > > > > > Baolin
> > > > > > > > Wang <baolin.wang@linux.alibaba.com>; Jyoti Bhayana
> > > > > > > > <jbhayana@google.com>; Chen-Yu Tsai <wens@csie.org>;
> > > > > > > > Orson
> > > > > > > > Zhai
> > > > > > > > <orsonzhai@gmail.com>
> > > > > > > > Subject: Re: [PATCH 13/15] iio: health: max30100: do
> > > > > > > > not
> > > > > > > > use
> > > > > > > > internal iio_dev
> > > > > > > > lock
> > > > > > > > 
> > > > > > > > [External]
> > > > > > > > 
> > > > > > > > Hi Nuno,
> > > > > > > > 
> > > > > > > > Nuno.Sa@analog.com wrote on Tue, 20 Sep 2022 12:44:08
> > > > > > > > +0000:
> > > > > > > >         
> > > > > > > > > > -----Original Message-----
> > > > > > > > > > From: Miquel Raynal <miquel.raynal@bootlin.com>
> > > > > > > > > > Sent: Tuesday, September 20, 2022 2:23 PM
> > > > > > > > > > To: Sa, Nuno <Nuno.Sa@analog.com>
> > > > > > > > > > Cc: linux-arm-kernel@lists.infradead.org; linux-
> > > > > > > > > >         
> > > > > > > > rockchip@lists.infradead.org;        
> > > > > > > > > > linux-amlogic@lists.infradead.org;
> > > > > > > > > > linux-imx@nxp.com;
> > > > > > > > > > linux-
> > > > > > > > > > iio@vger.kernel.org; Chunyan Zhang
> > > > > > > > > > <zhang.lyra@gmail.com>;        
> > > > > > > > Hennerich,        
> > > > > > > > > > Michael <Michael.Hennerich@analog.com>; Martin
> > > > > > > > > > Blumenstingl
> > > > > > > > > > <martin.blumenstingl@googlemail.com>; Sascha Hauer
> > > > > > > > > > <s.hauer@pengutronix.de>; Cixi Geng
> > > > > > > > > > <cixi.geng1@unisoc.com>;
> > > > > > > > > > Kevin
> > > > > > > > > > Hilman <khilman@baylibre.com>; Vladimir Zapolskiy
> > > > > > > > > > <vz@mleia.com>;
> > > > > > > > > > Pengutronix Kernel Team <kernel@pengutronix.de>;
> > > > > > > > > > Alexandru        
> > > > > > > > Ardelean        
> > > > > > > > > > <aardelean@deviqon.com>; Fabio Estevam
> > > > > > > > > > <festevam@gmail.com>;       
> > > > > > > > Andriy        
> > > > > > > > > > Tryshnivskyy <andriy.tryshnivskyy@opensynergy.com>;
> > > > > > > > > > Haibo
> > > > > > > > > > Chen
> > > > > > > > > > <haibo.chen@nxp.com>; Shawn Guo
> > > > > > > > > > <shawnguo@kernel.org>;
> > > > > > > > > > Hans
> > > > > > > > > > de
> > > > > > > > > > Goede <hdegoede@redhat.com>; Jerome Brunet        
> > > > > > > > <jbrunet@baylibre.com>;        
> > > > > > > > > > Heiko Stuebner <heiko@sntech.de>; Florian Boor
> > > > > > > > > > <florian.boor@kernelconcepts.de>; Regus, Ciprian
> > > > > > > > > > <Ciprian.Regus@analog.com>; Lars-Peter Clausen
> > > > > > > > > > <lars@metafoo.de>;        
> > > > > > > > Andy        
> > > > > > > > > > Shevchenko <andy.shevchenko@gmail.com>; Jonathan
> > > > > > > > > > Cameron
> > > > > > > > > > <jic23@kernel.org>; Neil Armstrong
> > > > > > > > > > <narmstrong@baylibre.com>;
> > > > > > > > > > Baolin
> > > > > > > > > > Wang <baolin.wang@linux.alibaba.com>; Jyoti Bhayana
> > > > > > > > > > <jbhayana@google.com>; Chen-Yu Tsai
> > > > > > > > > > <wens@csie.org>;
> > > > > > > > > > Orson
> > > > > > > > > > Zhai
> > > > > > > > > > <orsonzhai@gmail.com>
> > > > > > > > > > Subject: Re: [PATCH 13/15] iio: health: max30100:
> > > > > > > > > > do
> > > > > > > > > > not use
> > > > > > > > > > internal        
> > > > > > > > iio_dev        
> > > > > > > > > > lock
> > > > > > > > > > 
> > > > > > > > > > [External]
> > > > > > > > > > 
> > > > > > > > > > Hi Nuno,
> > > > > > > > > >        
> > > > > > > > > 
> > > > > > > > > Hi Miquel,
> > > > > > > > > 
> > > > > > > > > Thanks for reviewing...
> > > > > > > > >        
> > > > > > > > > > nuno.sa@analog.com wrote on Tue, 20 Sep 2022
> > > > > > > > > > 13:28:19
> > > > > > > > > > +0200:
> > > > > > > > > >        
> > > > > > > > > > > The pattern used in this device does not quite
> > > > > > > > > > > fit in
> > > > > > > > > > > the
> > > > > > > > > > > iio_device_claim_direct_mode() typical usage. In
> > > > > > > > > > > this
> > > > > > > > > > > case,
> > > > > > > > > > > iio_buffer_enabled() was being used not to
> > > > > > > > > > > prevent
> > > > > > > > > > > the raw
> > > > > > > > > > > access but        
> > > > > > > > to        
> > > > > > > > > > > allow it. Hence to get rid of the 'mlock' we need
> > > > > > > > > > > to:
> > > > > > > > > > > 
> > > > > > > > > > > 1. Use iio_device_claim_direct_mode() to check if
> > > > > > > > > > > direct
> > > > > > > > > > > mode can be
> > > > > > > > > > > claimed and if we can return -EINVAL (as the
> > > > > > > > > > > original
> > > > > > > > > > > code);
> > > > > > > > > > > 
> > > > > > > > > > > 2. Make sure that buffering is not disabled while
> > > > > > > > > > > doing a
> > > > > > > > > > > raw read. For
> > > > > > > > > > > that, we can make use of the local lock that
> > > > > > > > > > > already
> > > > > > > > > > > exists.
> > > > > > > > > > > 
> > > > > > > > > > > While at it, fixed a minor coding style
> > > > > > > > > > > complain...
> > > > > > > > > > > 
> > > > > > > > > > > Signed-off-by: Nuno Sá <nuno.sa@analog.com>
> > > > > > > > > > > ---
> > > > > > > > > > >  drivers/iio/health/max30100.c | 24
> > > > > > > > > > > +++++++++++++++++------
> > > > > > > > > > > -
> > > > > > > > > > >  1 file changed, 17 insertions(+), 7 deletions(-)
> > > > > > > > > > > 
> > > > > > > > > > > diff --git a/drivers/iio/health/max30100.c       
> > > > > > > > b/drivers/iio/health/max30100.c        
> > > > > > > > > > > index ad5717965223..aa494cad5df0 100644
> > > > > > > > > > > --- a/drivers/iio/health/max30100.c
> > > > > > > > > > > +++ b/drivers/iio/health/max30100.c
> > > > > > > > > > > @@ -185,8 +185,19 @@ static int
> > > > > > > > > > > max30100_buffer_postenable(struct        
> > > > > > > > > > iio_dev *indio_dev)        
> > > > > > > > > > >  static int max30100_buffer_predisable(struct
> > > > > > > > > > > iio_dev
> > > > > > > > > > > *indio_dev)
> > > > > > > > > > >  {
> > > > > > > > > > >         struct max30100_data *data =
> > > > > > > > > > > iio_priv(indio_dev);
> > > > > > > > > > > +       int ret;
> > > > > > > > > > > +
> > > > > > > > > > > +       /*
> > > > > > > > > > > +        * As stated in the comment in the
> > > > > > > > > > > read_raw()
> > > > > > > > > > > function, temperature
> > > > > > > > > > > +        * can only be acquired if the engine is
> > > > > > > > > > > running.
> > > > > > > > > > > As such the mutex
> > > > > > > > > > > +        * is used to make sure we do not power
> > > > > > > > > > > down
> > > > > > > > > > > while
> > > > > > > > > > > doing a        
> > > > > > > > > > temperature        
> > > > > > > > > > > +        * reading.
> > > > > > > > > > > +        */
> > > > > > > > > > > +       mutex_lock(&data->lock);
> > > > > > > > > > > +       ret = max30100_set_powermode(data,
> > > > > > > > > > > false);
> > > > > > > > > > > +       mutex_unlock(&data->lock);
> > > > > > > > > > > 
> > > > > > > > > > > -       return max30100_set_powermode(data,
> > > > > > > > > > > false);
> > > > > > > > > > > +       return ret;
> > > > > > > > > > >  }
> > > > > > > > > > > 
> > > > > > > > > > >  static const struct iio_buffer_setup_ops
> > > > > > > > > > > max30100_buffer_setup_ops        
> > > > > > > > = {        
> > > > > > > > > > > @@ -387,18 +398,17 @@ static int
> > > > > > > > > > > max30100_read_raw(struct
> > > > > > > > > > > iio_dev        
> > > > > > > > > > *indio_dev,        
> > > > > > > > > > >                  * Temperature reading can only
> > > > > > > > > > > be
> > > > > > > > > > > acquired
> > > > > > > > > > > while engine
> > > > > > > > > > >                  * is running
> > > > > > > > > > >                  */
> > > > > > > > > > > -               mutex_lock(&indio_dev->mlock);
> > > > > > > > > > > -
> > > > > > > > > > > -               if
> > > > > > > > > > > (!iio_buffer_enabled(indio_dev))
> > > > > > > > > > > +               if
> > > > > > > > > > > (!iio_device_claim_direct_mode(indio_dev))
> > > > > > > > > > > {        
> > > > > > > > > > 
> > > > > > > > > > I wonder if this line change here is really needed.
> > > > > > > > > > I
> > > > > > > > > > agree
> > > > > > > > > > the whole
> > > > > > > > > > construction looks like what
> > > > > > > > > > iio_device_claim_direct_mode()
> > > > > > > > > > does but in
> > > > > > > > > > practice I don't see the point of acquiring any
> > > > > > > > > > lock
> > > > > > > > > > here if
> > > > > > > > > > we just
> > > > > > > > > > release it no matter what happens right after.
> > > > > > > > > >        
> > > > > > > > > 
> > > > > > > > > I can see that this is odd (at the very least) but
> > > > > > > > > AFAIK,
> > > > > > > > > this
> > > > > > > > > is the only way
> > > > > > > > > to safely infer if buffering is enabled or not.
> > > > > > > > > iio_buffer_enabled() has no
> > > > > > > > > protection against someone concurrently
> > > > > > > > > enabling/disabling the
> > > > > > > > > buffer.        
> > > > > > > > 
> > > > > > > > Yes, but this is only relevant if you want to infer
> > > > > > > > that
> > > > > > > > the
> > > > > > > > "buffers
> > > > > > > > are enabled" and be sure that it cannot be otherwise
> > > > > > > > during
> > > > > > > > the
> > > > > > > > next
> > > > > > > > lines until you release the lock. Acquiring a lock,
> > > > > > > > doing
> > > > > > > > the if
> > > > > > > > and
> > > > > > > > then unconditionally releasing the lock, IMHO, does not
> > > > > > > > make any
> > > > > > > > sense
> > > > > > > > (but I'm not a locking guru) because when you enter the
> > > > > > > > else
> > > > > > > > clause,
> > > > > > > > you are not protected anyway, so in both cases all this
> > > > > > > > is
> > > > > > > > completely
> > > > > > > > racy.
> > > > > > > >         
> > > > > > > 
> > > > > > > Ahh crap, yes you are right... It is still racy since we
> > > > > > > can
> > > > > > > still
> > > > > > > try to read
> > > > > > > the temperature with the device powered off. I'm not
> > > > > > > really
> > > > > > > sure
> > > > > > > how to
> > > > > > > address this. One way could be to just use an internal
> > > > > > > control
> > > > > > > variable
> > > > > > > to reflect the device power state (set/clear on the
> > > > > > > buffer
> > > > > > > callbacks) and
> > > > > > > only use the local lock (completely ditching the call to
> > > > > > > iio_device_claim_direct_mode())...      
> > > > > > 
> > > > > > I tend to prefer this option than the one below.
> > > > > > 
> > > > > > I guess your implementation already prevents
> > > > > > buffer_predisable() to
> > > > > > run
> > > > > > thanks to the local lock being held during the operation.
> > > > > > Maybe
> > > > > > we
> > > > > > should just verify that buffers are enabled from within the
> > > > > > local
> > > > > > lock
> > > > > > being held instead of just acquiring it for the get_temp()
> > > > > > measure.
> > > > > > It
> > > > > > would probably solve the situation here.      
> > > > > > >       
> > > > > Not sure if I understood... You mean something like:
> > > > > 
> > > > > mutex_lock(&data->lock);
> > > > > if (!iio_buffer_enabled(indio_dev)) {
> > > > >         ret = -EINVAL;
> > > > > } else {
> > > > >         ret = max30100_get_temp(data, val);
> > > > >         if (!ret)
> > > > >                 ret = IIO_VAL_INT;
> > > > > 
> > > > > }
> > > > > mutex_unlock(&data->lock);
> > > > > 
> > > > > If so, I think this is still racy since we release the lock
> > > > > after
> > > > > the
> > > > > predisable which means we could still detect the buffers as
> > > > > enabled (in
> > > > > the above block) and try to get_temp on a powered down
> > > > > device.    
> > > > 
> > > > True.
> > > >   
> > > > > 
> > > > > Since we pretty much only care about the power state of the
> > > > > device (and
> > > > > we are using the buffering state to infer that AFAIU), I was
> > > > > thinking
> > > > > in something like:
> > > > > 
> > > > > 
> > > > > mutex_lock(&data->lock);
> > > > > if (!data->powered) {
> > > > >         ret = -EINVAL;
> > > > > } else {
> > > > >         ret = max30100_get_temp(data, val);
> > > > >         if (!ret)
> > > > >                 ret = IIO_VAL_INT;
> > > > > 
> > > > > }
> > > > > mutex_unlock(&data->lock);    
> > > > 
> > > > LGTM.  
> > > 
> > > A reference counted power up flag would probably work as we'd
> > > want to
> > > disable
> > > power only when the reference count goes to 0.  Note all checks
> > > of
> > > that flag
> > > would need to be done under the lock as well.
> > >   
> > 
> > Is there any way to enable a buffer more than once? Otherwise I'm
> > not
> > sure we really need a refcount... Any ways, your below approach
> > looks
> > good to me and surely easier.
> 
> Indeed can only have the buffer enabled once, but there may be a race
> with the disable of the buffer and this path. Hence need a ref count
> to detect when the count goes to zero, what ever the reason.
> A flag only covers one side of the race.
> 
> > > 
Sure, but I would "secure" it with the same lock on 'data->lock' and
the flag setting would be obviously under that lock (same on the
postenable side of things). IMO, I don't think we need to that far as
using refcounts in here. Anyways, I will give a try to the suggestion
you made in the other patch.

- Nuno Sá
> > 

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