Messages in this thread Patch in this message | | | Date | Thu, 7 Dec 2023 16:29:22 -0500 | Subject | Re: [PATCH v2 01/10] devm-helpers: introduce devm_mutex_init | From | Waiman Long <> |
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On 12/6/23 16:02, Waiman Long wrote: > On 12/6/23 14:55, Hans de Goede wrote: >> Hi, >> >> On 12/6/23 19:58, George Stark wrote: >>> Hello Hans >>> >>> Thanks for the review. >>> >>> On 12/6/23 18:01, Hans de Goede wrote: >>>> Hi George, >>>> >>>> On 12/4/23 19:05, George Stark wrote: >>>>> Using of devm API leads to 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() is >>>>> extended so introduce devm_mutex_init(). >>>>> >>>>> Signed-off-by: George Stark <gnstark@salutedevices.com> >>>>> --- >>>>> include/linux/devm-helpers.h | 18 ++++++++++++++++++ >>>>> 1 file changed, 18 insertions(+) >>>>> >>>>> diff --git a/include/linux/devm-helpers.h >>>>> b/include/linux/devm-helpers.h >>>>> index 74891802200d..2f56e476776f 100644 >>>>> --- a/include/linux/devm-helpers.h >>>>> +++ b/include/linux/devm-helpers.h >>>>> @@ -76,4 +76,22 @@ static inline int devm_work_autocancel(struct >>>>> device *dev, >>>>> return devm_add_action(dev, devm_work_drop, w); >>>>> } >>>>> +static inline void devm_mutex_release(void *res) >>>>> +{ >>>>> + mutex_destroy(res); >>>>> +} >>>>> + >>>>> +/** >>>>> + * devm_mutex_init - Resource-managed mutex initialization >>>>> + * @dev: Device which lifetime work is bound to >>>>> + * @lock: Pointer to a mutex >>>>> + * >>>>> + * Initialize mutex which is automatically destroyed when driver >>>>> is detached. >>>>> + */ >>>>> +static inline int devm_mutex_init(struct device *dev, struct >>>>> mutex *lock) >>>>> +{ >>>>> + mutex_init(lock); >>>>> + return devm_add_action_or_reset(dev, devm_mutex_release, lock); >>>>> +} >>>>> + >>>>> #endif >>>> mutex_destroy() only actually does anything if CONFIG_DEBUG_MUTEXES >>>> is set, otherwise it is an empty inline-stub. >>>> >>>> Adding a devres resource to the device just to call an empty inline >>>> stub which is a no-op seems like a waste of resources. IMHO it >>>> would be better to change this to: >>>> >>>> static inline int devm_mutex_init(struct device *dev, struct mutex >>>> *lock) >>>> { >>>> mutex_init(lock); >>>> #ifdef CONFIG_DEBUG_MUTEXES >>>> return devm_add_action_or_reset(dev, devm_mutex_release, lock); >>>> #else >>>> return 0; >>>> #endif >>>> } >>>> >>>> To avoid the unnecessary devres allocation when >>>> CONFIG_DEBUG_MUTEXES is not set. >>> Honestly saying I don't like unnecessary devres allocation either >>> but the proposed approach has its own price: >>> >>> 1) we'll have more than one place with branching if mutex_destroy is >>> empty or not using indirect condition. If suddenly mutex_destroy is >>> extended for non-debug code (in upstream branch or e.g. by someone >>> for local debug) than there'll be a problem. >>> >>> 2) If mutex_destroy is empty or not depends on CONFIG_PREEMPT_RT >>> option too. When CONFIG_PREEMPT_RT is on mutex_destroy is always empty. >>> >>> As I see it only the mutex interface (mutex.h) has to say definitely >>> if mutex_destroy must be called. Probably we could add some define >>> to include/linux/mutex.h,like IS_MUTEX_DESTROY_REQUIRED and declare >>> it near mutex_destroy definition itself. >> That (a IS_MUTEX_DESTROY_REQUIRED define) is an interesting idea. >> Lets see for v3 if the mutex maintainers will accept that and if not >> then I guess we will just need to live with the unnecessary devres >> allocation. > > The purpose of calling mutex_destroy() is to mark a mutex as being > destroyed so that any subsequent call to mutex_lock/unlock will cause > a warning to be printed when CONFIG_DEBUG_MUTEXES is defined. I would > not say that mutex_destroy() is required. Rather it is a nice to have > for catching programming error.
OTOH, one thing that we can probably do in mutex.h is something like
diff --git a/include/linux/mutex.h b/include/linux/mutex.h index a33aa9eb9fc3..7db7862de3f1 100644 --- a/include/linux/mutex.h +++ b/include/linux/mutex.h @@ -83,6 +83,9 @@ struct mutex {
extern void mutex_destroy(struct mutex *lock);
+/* mutex_destroy() is a real function, not a NOP */ +#define mutex_destroy mutex_destroy + #else
---------------------------------------- Now in some devm files, you can use the absense/presence of mutex_destroy macro to decide on what to do.
Cheers, Longman
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