[PATCH v4 02/10] locking: introduce devm_mutex_init

[Christophe Leroy]

Le 17/12/2023 à 02:05, George Stark a écrit :
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> 
> Hello Christophe
> 
> On 12/15/23 08:46, Christophe Leroy wrote:
>>
>>
>> Le 14/12/2023 à 22:48, Waiman Long a écrit :
>>> On 12/14/23 14:53, Christophe Leroy wrote:
>>>>
>>>> Le 14/12/2023 à 19:48, Waiman Long a écrit :
>>>>> On 12/14/23 12:36, George Stark wrote:
>>>>>> Using of devm API leads to a certain order of releasing resources.
>>>>>> So all dependent resources which are not devm-wrapped should be 
>>>>>> deleted
>>>>>> with respect to devm-release order. Mutex is one of such objects that
>>>>>> often is bound to other resources and has no own devm wrapping.
>>>>>> Since mutex_destroy() actually does nothing in non-debug builds
>>>>>> frequently calling mutex_destroy() is just ignored which is safe for
>>>>>> now
>>>>>> but wrong formally and can lead to a problem if mutex_destroy() 
>>>>>> will be
>>>>>> extended so introduce devm_mutex_init()
>>>>>>
>>>>>> Signed-off-by: George Stark <gnstark at salutedevices.com>
>>>>>> ---
>>>>>>     include/linux/mutex.h        | 23 +++++++++++++++++++++++
>>>>>>     kernel/locking/mutex-debug.c | 22 ++++++++++++++++++++++
>>>>>>     2 files changed, 45 insertions(+)
>>>>>>
>>>>>> diff --git a/include/linux/mutex.h b/include/linux/mutex.h
>>>>>> index a33aa9eb9fc3..ebd03ff1ef66 100644
>>>>>> --- a/include/linux/mutex.h
>>>>>> +++ b/include/linux/mutex.h
>>>>>> @@ -21,6 +21,8 @@
>>>>>>     #include <linux/debug_locks.h>
>>>>>>     #include <linux/cleanup.h>
>>>>>> +struct device;
>>>>>> +
>>>>>>     #ifdef CONFIG_DEBUG_LOCK_ALLOC
>>>>>>     # define __DEP_MAP_MUTEX_INITIALIZER(lockname)            \
>>>>>>             , .dep_map = {                    \
>>>>>> @@ -127,6 +129,20 @@ extern void __mutex_init(struct mutex *lock,
>>>>>> const char *name,
>>>>>>      */
>>>>>>     extern bool mutex_is_locked(struct mutex *lock);
>>>>>> +#ifdef CONFIG_DEBUG_MUTEXES
>>>>>> +
>>>>>> +int devm_mutex_init(struct device *dev, struct mutex *lock);
>>>>> Please add "extern" to the function declaration to be consistent with
>>>>> other functional declarations in mutex.h.
>>>> 'extern' is pointless and deprecated on function prototypes. Already
>>>> having some is not a good reason to add new ones, errors from the past
>>>> should be avoided nowadays. With time they should all disappear so 
>>>> don't
>>>> add new ones.
>>> Yes, "extern" is optional. It is just a suggestion and I am going to
>>> argue about that.
>>
>> FWIW, note that when you perform a strict check with checkpatch.pl, you
>> get a warning for that:
>>
>> $ ./scripts/checkpatch.pl --strict -g HEAD
>> CHECK: extern prototypes should be avoided in .h files
>> #56: FILE: include/linux/mutex.h:131:
>> +extern int devm_mutex_init(struct device *dev, struct mutex *lock);
>>
>> total: 0 errors, 0 warnings, 1 checks, 99 lines checked
> 
> This is ambiguous situation about extern. It's deprecated and useless on
> one hand but harmless. And those externs will not disappear by themself
> - it'll be one patch that clean them all at once (in one header at
> least) so one more extern will not alter the overall picture.

That kind of cleanup patch bomb is a nightmare for backporting, so if it 
happens one day it should be as light as possible, hence the importance 
to not add new ones and remove existing one everytime you modify or move 
a line including it for whatever reason.

> 
> On the other hand if we manage to place devm_mutex_init near
> mutex_destroy then we'll have:
> 
> int devm_mutex_init(struct device *dev, struct mutex *lock);
> extern void mutex_destroy(struct mutex *lock);

I sent you an alternative proposal that avoids duplication of the static 
inline version of devm_mutex_init(). If you agree with it just take it 
into your series and that question will vanish.

> 
> and it raises questions and does not look very nice.

If you look at linux/mm.h there are plenty of them anyway, so why do 
different ? For an exemple look at 
https://elixir.bootlin.com/linux/v6.7-rc4/source/include/linux/mm.h#L2372


Christophe