Hello Christophe
On 12/7/23 01:37, Christophe Leroy wrote:
Le 06/12/2023 à 23:14, Christophe Leroy a écrit :
Le 06/12/2023 à 19:58, George Stark a écrit :
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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@xxxxxxxxxxxxxxxxx>
---
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.
I tried to put devm_mutex_init itself in mutex.h and it could've helped
too but it's not the place for devm API.
What do you mean by "it's not the place for devm API" ?
If you do a 'grep devm_ include/linux/' you'll find devm_ functions in
almost 100 .h files. Why wouldn't mutex.h be the place for
devm_mutex_init() ?
mutex.h's maintainers believe so.
https://lore.kernel.org/lkml/070c174c-057a-46de-ae8e-836e9e20eceb@xxxxxxxxxxxxxxxxx/T/#mb42e1d7760816b0cedd3130e08f29690496b5ac2
Looking at it closer, I have the feeling that you want to do similar to
devm_gpio_request() in linux/gpio.h :
In linux/mutex.h, add a prototype for devm_mutex_init() when
CONFIG_DEBUG_MUTEXES is defined and an empty static inline otherwise.
Then define devm_mutex_init() in kernel/locking/mutex-debug.c
Yes, this would be almost perfect decision. BTW just as in linux/gpio.h
we wouldn't have to include whole "linux/device.h" into mutex.h, only
add forward declaration of struct device;
Wouldn't that work ?
Christophe
--
Best regards
George
