linux-kernel - Re: [PATCH 1/5] debugobjects: Fix the misuse of global variables in fill

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Message-ID: <13d2be50-4a52-7cf0-8325-65435ad47a62@huawei.com>
Date: Tue, 3 Sep 2024 10:16:20 +0800
From: "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To: Thomas Gleixner <tglx@...utronix.de>, Andrew Morton
	<akpm@...ux-foundation.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/5] debugobjects: Fix the misuse of global variables in
 fill_pool()



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.
> 
> 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;

 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