linux-kernel - Re: [PATCH v4 02/10] locking: introduce devm_mutex

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Message-ID: <d160443b-b973-4162-9900-95c04e62cd65@salutedevices.com>
Date: Mon, 18 Dec 2023 16:26:30 +0300
From: George Stark <gnstark@...utedevices.com>
To: Christophe Leroy <christophe.leroy@...roup.eu>, Waiman Long
	<longman@...hat.com>, "andy.shevchenko@...il.com"
	<andy.shevchenko@...il.com>, "pavel@....cz" <pavel@....cz>, "lee@...nel.org"
	<lee@...nel.org>, "vadimp@...dia.com" <vadimp@...dia.com>,
	"mpe@...erman.id.au" <mpe@...erman.id.au>, "npiggin@...il.com"
	<npiggin@...il.com>, "hdegoede@...hat.com" <hdegoede@...hat.com>,
	"mazziesaccount@...il.com" <mazziesaccount@...il.com>, "peterz@...radead.org"
	<peterz@...radead.org>, "mingo@...hat.com" <mingo@...hat.com>,
	"will@...nel.org" <will@...nel.org>, "boqun.feng@...il.com"
	<boqun.feng@...il.com>, "nikitos.tr@...il.com" <nikitos.tr@...il.com>
CC: "linux-leds@...r.kernel.org" <linux-leds@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"kernel@...utedevices.com" <kernel@...utedevices.com>
Subject: Re: [PATCH v4 02/10] locking: introduce devm_mutex_init

Hello Christophe


On 12/17/23 12:31, Christophe Leroy wrote:

...
>>>>>>> ---
>>>>>>>      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.

Thanks for that patch by the way. The only comment is that moving 
mutex_destroy
should be done in a separate patch IMO.
Waiman Long proposed such a refactoring here:
https://lore.kernel.org/lkml/20231216013656.1382213-2-longman@redhat.com/T/

With this patch adding devm_mutex_init would be straightforward.

>>
>> 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
Oh, I see. Ok, I don't have any more arguments against removing extern.
We'll see what mutex.h maintainers decide.

> 
> Christophe

-- 
Best regards
George