Re: [PATCH 1/5] debugobjects: Fix the misuse of global variables in fill_pool()

["Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>]

On 2024/9/3 11:22, Leizhen (ThunderTown) wrote:
> 
> 
> On 2024/9/3 10:16, Leizhen (ThunderTown) wrote:
>>
>>
>> On 2024/9/3 0:22, Thomas Gleixner wrote:
>>> On Mon, Sep 02 2024 at 22:05, Zhen Lei wrote:
>>>
>>>> The global variable 'obj_pool_min_free' records the lowest historical
>>>> value of the number of nodes in the global list 'obj_pool', instead of
>>>> being used as the lowest threshold value. This may be a mistake and
>>>
>>> Maybe? It's either a bug or not.
>>
>> Yes, it's a bug, but I'm just learning this module, so I'm not confident.
>>
>>>
>>>> should be replaced with variable 'debug_objects_pool_min_level'.
>>>
>>> And if it's a bug then it has to be replaced.
>>
>> OK, I will update the commit message in V2.
>>
>>>
>>> This misses another minor issue:
>>>
>>> static int			obj_pool_min_free = ODEBUG_POOL_SIZE;
>>>
>>> static int  __data_racy		debug_objects_pool_min_level __read_mostly
>>> 				= ODEBUG_POOL_MIN_LEVEL;
>>>
>>> As debug_objects_pool_min_level is the minimum level to keep around and
>>> obj_pool_min_free is a statistics mechanism, __data_racy is misplaced
>>> too. The variables should swap their position, because
>>> debug_objects_pool_min_level is functional, but obj_pool_min_free is
>>> pure stats.
> 
> This principle covers a large number of variables. It should be sorted
> by the variable name before. It's better not to change the position this time.
> 
>>>
>>> Also debug_objects_pool_min_level and debug_objects_pool_size should
>>> be __ro_after_init.
>>
>> OK, How about modifying it like this? I further removed the __data_racy from
>> debug_objects_pool_min_level and debug_objects_pool_size, because __ro_after_init.
>>
>>
>> diff --git a/lib/debugobjects.c b/lib/debugobjects.c
>> index d3845705db955fa..816d3d968cd9f14 100644
>> --- a/lib/debugobjects.c
>> +++ b/lib/debugobjects.c
>> @@ -70,7 +70,8 @@ static HLIST_HEAD(obj_to_free);
>>   * made at debug_stats_show(). Both obj_pool_min_free and obj_pool_max_used
>>   * can be off.
>>   */
>> -static int                     obj_pool_min_free = ODEBUG_POOL_SIZE;
>> +static int __ro_after_init     debug_objects_pool_size = ODEBUG_POOL_SIZE;
>> +static int __ro_after_init     debug_objects_pool_min_level = ODEBUG_POOL_MIN_LEVEL;
>>  static int                     obj_pool_free = ODEBUG_POOL_SIZE;
>>  static int                     obj_pool_used;
>>  static int                     obj_pool_max_used;
>> @@ -84,10 +85,7 @@ static int __data_racy                       debug_objects_fixups __read_mostly;
>>  static int __data_racy                 debug_objects_warnings __read_mostly;
>>  static int __data_racy                 debug_objects_enabled __read_mostly
>>                                         = CONFIG_DEBUG_OBJECTS_ENABLE_DEFAULT;
>> -static int __data_racy                 debug_objects_pool_size __read_mostly
>> -                                       = ODEBUG_POOL_SIZE;
>> -static int __data_racy                 debug_objects_pool_min_level __read_mostly
>> -                                       = ODEBUG_POOL_MIN_LEVEL;
>> +static int __data_racy                 obj_pool_min_free = ODEBUG_POOL_SIZE;

Sorry, I rechecked it again. After this patch, obj_pool_min_free is referenced in the
same way as obj_pool_max_used. The only race point is located in debug_stats_show().
However, this reference point does not need to be included in the race analysis. So
there is no need to add __data_racy for obj_pool_min_free.

The final modification is as follows:

diff --git a/lib/debugobjects.c b/lib/debugobjects.c
index d3845705db955fa..721e207f85b6be7 100644
--- a/lib/debugobjects.c
+++ b/lib/debugobjects.c
@@ -84,9 +84,9 @@ static int __data_racy                        debug_objects_fixups __read_mostly;
 static int __data_racy                 debug_objects_warnings __read_mostly;
 static int __data_racy                 debug_objects_enabled __read_mostly
                                        = CONFIG_DEBUG_OBJECTS_ENABLE_DEFAULT;
-static int __data_racy                 debug_objects_pool_size __read_mostly
+static int                             debug_objects_pool_size __ro_after_init
                                        = ODEBUG_POOL_SIZE;
-static int __data_racy                 debug_objects_pool_min_level __read_mostly
+static int                             debug_objects_pool_min_level __ro_after_init
                                        = ODEBUG_POOL_MIN_LEVEL;

 static const struct debug_obj_descr *descr_test  __read_mostly;


>>
>>  static const struct debug_obj_descr *descr_test  __read_mostly;
>>  static struct kmem_cache       *obj_cache __ro_after_init;
>>
>>
>>>
>>>> Fixes: d26bf5056fc0 ("debugobjects: Reduce number of pool_lock acquisitions in fill_pool()")
>>>> Fixes: 36c4ead6f6df ("debugobjects: Add global free list and the counter")
>>>
>>> Nice detective work!
>>>
>>> Thanks,
>>>
>>>         tglx
>>> .
>>>
>>
> 

-- 
Regards,
  Zhen Lei