Re: [PATCH v4 02/10] locking: introduce devm_mutex_init

[George Stark]

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@xxxxxxxxxx/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