linux-kernel - Re: [PATCH] regulator: simplify locking

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Date:   Mon, 10 Aug 2020 00:30:30 +0200
From:   Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] regulator: simplify locking

On Mon, Aug 10, 2020 at 12:40:04AM +0300, Dmitry Osipenko wrote:
> 10.08.2020 00:16, Michał Mirosław пишет:
> > Simplify regulator locking by removing locking around locking. rdev->ref
> > is now accessed only when the lock is taken. The code still smells fishy,
> > but now its obvious why.
> > 
> > Fixes: f8702f9e4aa7 ("regulator: core: Use ww_mutex for regulators locking")
> > Signed-off-by: Michał Mirosław <mirq-linux@...e.qmqm.pl>
> > ---
> >  drivers/regulator/core.c         | 37 ++++++--------------------------
> >  include/linux/regulator/driver.h |  1 -
> >  2 files changed, 6 insertions(+), 32 deletions(-)
> > 
> > diff --git a/drivers/regulator/core.c b/drivers/regulator/core.c
> > index 9e18997777d3..b0662927487c 100644
> > --- a/drivers/regulator/core.c
> > +++ b/drivers/regulator/core.c
> > @@ -45,7 +45,6 @@
> >  	pr_debug("%s: " fmt, rdev_get_name(rdev), ##__VA_ARGS__)
> >  
> >  static DEFINE_WW_CLASS(regulator_ww_class);
> > -static DEFINE_MUTEX(regulator_nesting_mutex);
> >  static DEFINE_MUTEX(regulator_list_mutex);
> >  static LIST_HEAD(regulator_map_list);
> >  static LIST_HEAD(regulator_ena_gpio_list);
> > @@ -150,32 +149,13 @@ static bool regulator_ops_is_valid(struct regulator_dev *rdev, int ops)
> >  static inline int regulator_lock_nested(struct regulator_dev *rdev,
> >  					struct ww_acquire_ctx *ww_ctx)
> >  {
> > -	bool lock = false;
> >  	int ret = 0;
> >  
> > -	mutex_lock(&regulator_nesting_mutex);
> > +	if (ww_ctx || !mutex_trylock_recursive(&rdev->mutex.base))
> 
> Have you seen comment to the mutex_trylock_recursive()?
> 
> https://elixir.bootlin.com/linux/v5.8/source/include/linux/mutex.h#L205
> 
>  * This function should not be used, _ever_. It is purely for hysterical GEM
>  * raisins, and once those are gone this will be removed.
> 
> I knew about this function and I don't think it's okay to use it, hence
> this is why there is that "nesting_mutex" and "owner" checking.
> 
> If you disagree, then perhaps you should make another patch to remove
> the stale comment to trylock_recursive().

I think that reimplementing the function just to not use it is not the
right solution. The whole locking protocol is problematic and this patch
just uncovers one side of it.

Best Regards,
Michał Mirosław