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

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Date:   Mon, 10 Aug 2020 03:21:47 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Michał Mirosław <mirq-linux@...e.qmqm.pl>
Cc:     Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] regulator: simplify locking

10.08.2020 01:30, Michał Mirosław пишет:
> 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.

It's not clear to me what is uncovered, the ref_cnt was always accessed
under lock. Could you please explain in a more details?

Would be awesome if you could improve the code, but then you should
un-deprecate the trylock_recursive() before making use of it. Maybe
nobody will mind and it all will be good in the end.