Re: [PATCH v2 0/8] Remove adis_initial_startup usage

[Jonathan Cameron <jic23@kernel.org>]

On Thu, 3 Nov 2022 12:35:31 +0000
"Sa, Nuno" <Nuno.Sa@analog.com> wrote:

> > From: Ramona Bolboaca <ramona.bolboaca@analog.com>
> > Sent: Thursday, November 3, 2022 9:09 AM
> > To: jic23@kernel.org; linux-iio@vger.kernel.org; linux-
> > kernel@vger.kernel.org
> > Cc: Bolboaca, Ramona <Ramona.Bolboaca@analog.com>
> > Subject: [PATCH v2 0/8] Remove adis_initial_startup usage
> > 
> > 
> > Remove 'adis_initial_startup()' usage due to the fact that it leads to a
> > deadlock.
> > The same mutex is acquired twice, without releasing it, once inside
> > 'adis_initial_startup()' and once inside 'adis_enable_irq()'.
> > Instead of 'adis_initial_startup()', use '__adis_initial_startup()'.
> > 
> > Ramona Bolboaca (8):
> >   iio: accel: adis16201: Fix deadlock in probe
> >   iio: accel: adis16209: Fix deadlock in probe
> >   iio: gyro: adis16136: Fix deadlock in probe
> >   iio: gyro: adis16260: Fix deadlock in probe
> >   iio: imu: adis16400: Fix deadlock in probe
> >   staging: iio: accel: adis16203: Fix deadlock in probe
> >   staging: iio: accel: adis16240: Fix deadlock in probe
> >   iio: imu: adis: Remove adis_initial_startup function
> > 
> >  drivers/iio/accel/adis16201.c         |  2 +-
> >  drivers/iio/accel/adis16209.c         |  2 +-
> >  drivers/iio/gyro/adis16136.c          |  2 +-
> >  drivers/iio/gyro/adis16260.c          |  2 +-
> >  drivers/iio/imu/adis16400.c           |  2 +-
> >  drivers/staging/iio/accel/adis16203.c |  2 +-
> >  drivers/staging/iio/accel/adis16240.c |  2 +-
> >  include/linux/iio/imu/adis.h          | 12 ------------
> >  8 files changed, 7 insertions(+), 19 deletions(-)
> >   
> 
> You could have placed your v2 changelog in the cover letter.
> Moreover it's the same for all patches... Anyways: 
> 
> Reviewed-by: Nuno Sá <nuno.sa@analog.com>

This feels a little backwards.  Normally we'd expect the
outer function to take the lock and the inner call to not
do so.  Now it's fine to not take the lock here at all because
the outer function call is in probe anyway, before we reach
the point where there should be an concurrency.

I wonder if we should instead do this by having
an unlocked __adis_enable_irq() that is always called
by __adis_initial_startup().  That would be the fix that
then needs backporting.

Switching the calls from adis_initial_startup() to
__adis_initial_startup() would then just be a trivial
optimization to not take locks before they should ever matter.

This all hinges on my assumption that the lock isn't useful.
Am I right on that?

Jonathan


> 
> - Nuno Sá