On 12/6/23 16:02, Waiman Long wrote:

On 12/6/23 14:55, Hans de Goede wrote:

quoted

Hi,

On 12/6/23 19:58, George Stark wrote:

quoted

Hello Hans

Thanks for the review.

On 12/6/23 18:01, Hans de Goede wrote:

quoted

Hi George,

On 12/4/23 19:05, George Stark wrote:

quoted

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 <redacted>
---
   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