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

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Date:   Mon, 10 Aug 2020 00:40:04 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] regulator: simplify locking

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().