Re: [PATCH v5 02/10] locking/mutex: introduce devm_mutex_init

[Waiman Long]

On 3/7/24 04:56, Marek Behún wrote:
> On Thu, Mar 07, 2024 at 05:40:26AM +0300, 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@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> > ---
> >   Hello Christophe. Hope you don't mind I put you SoB tag because you helped alot
> >   to make this patch happen.
> >
> >   include/linux/mutex.h        | 13 +++++++++++++
> >   kernel/locking/mutex-debug.c | 22 ++++++++++++++++++++++
> >   2 files changed, 35 insertions(+)
> >
> > diff --git a/include/linux/mutex.h b/include/linux/mutex.h
> > index f7611c092db7..9bcf72cb941a 100644
> > --- a/include/linux/mutex.h
> > +++ b/include/linux/mutex.h
> > @@ -22,6 +22,8 @@
> >   #include <linux/cleanup.h>
> >   #include <linux/mutex_types.h>
> >
> > +struct device;
> > +
> >   #ifdef CONFIG_DEBUG_LOCK_ALLOC
> >   # define __DEP_MAP_MUTEX_INITIALIZER(lockname)			\
> >   		, .dep_map = {					\
> > @@ -115,10 +117,21 @@ do {							\
> >
> >   #ifdef CONFIG_DEBUG_MUTEXES
> >
> > +int devm_mutex_init(struct device *dev, struct mutex *lock);
> >   void mutex_destroy(struct mutex *lock);
> >
> >   #else
> >
> > +static inline int devm_mutex_init(struct device *dev, struct mutex *lock)
> > +{
> > +	/*
> > +	 * since mutex_destroy is nop actually there's no need to register it
> > +	 * in devm subsystem.
> > +	 */
> > +	mutex_init(lock);
> > +	return 0;
> > +}
> > +
> >   static inline void mutex_destroy(struct mutex *lock) {}
> >
> >   #endif
> > diff --git a/kernel/locking/mutex-debug.c b/kernel/locking/mutex-debug.c
> > index bc8abb8549d2..c9efab1a8026 100644
> > --- a/kernel/locking/mutex-debug.c
> > +++ b/kernel/locking/mutex-debug.c
> > @@ -19,6 +19,7 @@
> >   #include <linux/kallsyms.h>
> >   #include <linux/interrupt.h>
> >   #include <linux/debug_locks.h>
> > +#include <linux/device.h>
> >
> >   #include "mutex.h"
> >
> > @@ -104,3 +105,24 @@ void mutex_destroy(struct mutex *lock)
> >   }
> >
> >   EXPORT_SYMBOL_GPL(mutex_destroy);
> > +
> > +static void devm_mutex_release(void *res)
> > +{
> > +	mutex_destroy(res);
> > +}
> > +
> > +/**
> > + * devm_mutex_init - Resource-managed mutex initialization
> > + * @dev:	Device which lifetime mutex is bound to
> > + * @lock:	Pointer to a mutex
> > + *
> > + * Initialize mutex which is automatically destroyed when the driver is detached.
> > + *
> > + * Returns: 0 on success or a negative error code on failure.
> > + */
> > +int devm_mutex_init(struct device *dev, struct mutex *lock)
> > +{
> > +	mutex_init(lock);
> > +	return devm_add_action_or_reset(dev, devm_mutex_release, lock);
> > +}
> > +EXPORT_SYMBOL_GPL(devm_mutex_init);
> Hi George,
>
> look at
> https://lore.kernel.org/lkml/7013bf9e-2663-4613-ae61-61872e81355b@xxxxxxxxxx/
> where Matthew and Hans explain that devm_mutex_init needs to be a macro
> because of the static lockdep key.
>
> so this should be something like:
>
> static inline int __devm_mutex_init(struct device *dev, struct mutex *mutex,
> 				    const char *name,
> 				    struct lock_class_key *key)
> {
> 	__mutex_init(mutex, name, key);
> 	return devm_add_action_or_reset(dev, devm_mutex_release, mutex);
> }
>
> #define devm_mutex_init(dev, mutex)				\
> do {								\
> 	static struct lock_class_key __key;			\
> 								\
> 	__devm_mutex_init(dev, (mutex), #mutex, &__key);	\
> } while (0);
>
>
> Marek

Making devm_mutex_init() a function will make all the devm_mutex share 
the same lockdep key. Making it a macro will make each caller of 
devm_mutex_init() have a distinct lockdep key. It all depends on whether 
all the devm_mutexes have the same lock usage pattern or not and whether 
it is possible for one devm_mutex to be nested inside another. So either 
way can be fine depending on the mutex usage pattern. My suggestion is 
to use a function, if possible, unless it will cause a false positive 
lockdep splat as there is a limit on the maximum # of lockdep keys that 
can be used.

Cheers,
Longman