On Thu, 7 Mar 2024 08:39:46 -0500
Waiman Long <longman@xxxxxxxxxx> wrote:
> 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.
devm_mutex_init() should behave like other similar function
initializing stuff with resource management. I.e. it should behave like
mutex_init(), but with resource management.
mutex_init() is a macro generating static lockdep key for each instance,
so devm_mutex_init() should also generate static lockdep key for each
instance.
Marek
