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@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.
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
