netdev - Re: [PATCH v3 net-next 01/23] lib: add reference counting tracking infrastructure

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Date:   Wed, 8 Dec 2021 16:04:16 +0100
From:   Andrzej Hajda <andrzej.hajda@...el.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
CC:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>, Greg KH <greg@...ah.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v3 net-next 01/23] lib: add reference counting tracking
 infrastructure


On 08.12.2021 15:27, Dmitry Vyukov wrote:
> On Wed, 8 Dec 2021 at 15:09, Andrzej Hajda <andrzej.hajda@...el.com> wrote:
>> Hi Eric,
>>
>>
>> I've spotted this patchset thanks to LWN, anyway it was merged very
>> quickly, I think it missed more broader review.
>>
>> As the patch touches kernel lib I have added few people who could be
>> interested.
>>
>>
>> On 05.12.2021 05:21, Eric Dumazet wrote:
>>> From: Eric Dumazet <edumazet@...gle.com>
>>>
>>> It can be hard to track where references are taken and released.
>>>
>>> In networking, we have annoying issues at device or netns dismantles,
>>> and we had various proposals to ease root causing them.
>>>
>>> This patch adds new infrastructure pairing refcount increases
>>> and decreases. This will self document code, because programmers
>>> will have to associate increments/decrements.
>>>
>>> This is controled by CONFIG_REF_TRACKER which can be selected
>>> by users of this feature.
>>>
>>> This adds both cpu and memory costs, and thus should probably be
>>> used with care.
>>>
>>> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
>>> Reviewed-by: Dmitry Vyukov <dvyukov@...gle.com>
>>> ---
>>
>> Life is surprising, I was working on my own framework, solving the same
>> issue, with intention to publish it in few days :)
>>
>>
>> My approach was little different:
>>
>> 1. Instead of creating separate framework I have extended debug_objects.
>>
>> 2. There were no additional fields in refcounted object and trackers -
>> infrastructure of debug_objects was reused - debug_objects tracked both
>> pointers of refcounted object and its users.
>>
>> Have you considered using debug_object? it seems to be good place to put
>> it there, I am not sure about performance differences.
> Hi Andrzej,
>
> How exactly did you do it? Do you have a link to your patch?
> There still should be something similar to `struct ref_tracker` in
> this patch, right? Or how do you match decrements with increments and
> understand when a double-decrement happens?


User during taking/dropping reference should pass pointer of the object 
who uses the reference (user).

And this pointer is tracked by debug_objects:

- on taking reference: the pointer is added to in-framework array 
associated with that reference,

- on dropping reference: framework checks if the pointer is in the 
array/quarantine, and the bug is accordingly reported.

- on destroying reference: bug is reported if users array is not empty,

- on taking/dropping reference to non-existing/destroyed object: bug is 
reported.


So instead of adding tracker field to user and passing it to the 
framework, address of the user itself is passed to the framework.


Regards

Andrzej