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

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Date:   Mon, 10 Aug 2020 02:59:27 +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 03:21:47AM +0300, Dmitry Osipenko wrote:
> 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.

I'm not sure why the framework wants recursive locking? If only for the
coupling case, then ww_mutex seems the right direction to replace it:
while walking the graph it will detect entering the same node
a second time. But this works only during the locking transaction (with
ww_context != NULL). Allowing recursive regulator_lock() outside of it
seems inviting trouble.

Best Regards,
Michał Mirosław