Re: [PATCH RFC 2/3] locking/rwsem: clear reader-owner on unlock to reduce false positives

[Lance Yang <lance.yang@linux.dev>]

On 2025/6/24 14:13, Masami Hiramatsu (Google) wrote:
> On Tue, 24 Jun 2025 13:02:31 +0800
> Lance Yang <lance.yang@linux.dev> wrote:
> 
>>
>>
>> On 2025/6/24 11:53, Masami Hiramatsu (Google) wrote:
>>> On Tue, 24 Jun 2025 09:44:55 +0800
>>> Lance Yang <lance.yang@linux.dev> wrote:
>>>
>>>>
>>>>
>>>> On 2025/6/24 08:26, Masami Hiramatsu (Google) wrote:
>>>>> On Thu, 12 Jun 2025 12:19:25 +0800
>>>>> Lance Yang <ioworker0@gmail.com> wrote:
>>>>>
>>>>>> From: Lance Yang <lance.yang@linux.dev>
>>>>>>
>>>>>> When CONFIG_DETECT_HUNG_TASK_BLOCKER is enabled, a stale owner pointer in a
>>>>>> reader-owned rwsem can lead to false positives in blocker tracking.
>>>>>>
>>>>>> To mitigate this, let’s try to clear the owner field on unlock, as a NULL
>>>>>> owner is better than a stale one for diagnostics.
>>>>>
>>>>> Can we merge this to [PATCH 1/3]? It seems that you removed #ifdef and
>>>>> remove it. This means in anyway we need the feature enabled by DEBUG_RWSEMS.
>>>>
>>>> Thanks for the feedback! I see your point about the dependency ;)
>>>>
>>>> Personlly, I'd perfer to keep them separate. The reasoning is that
>>>> they addreess two distinct things, and I think splitting them makes
>>>> this series clearer and easier to review ;)
>>>>
>>>> Patch #1 focuses on "ownership tracking": Its only job is to make
>>>> the existing owner-related helpers (rwsem_owner(), is_rwsem_reader_owned())
>>>> globally available when blocker tracking is enabled.
>>>>
>>>> Patch #2, on the other hand, is about "reader-owner cleanup": It
>>>> introduces a functional change to the unlock path, trying to clear
>>>> the owner field for reader-owned rwsems.
>>>
>>> But without clearing the owner, the owner information can be
>>> broken, right? Since CONFIG_DEBUG_RWSEMS is working as it is,
>>
>> You're right, the owner info would be broken without the cleanup logic
>> in patch #2. But ...
>>
>>> I think those cannot be decoupled. For example, comparing the
>>> result of both DETECT_HUNG_TASK_BLOCKER and DEBUG_RWSEMS are
>>> enabled and only DETECT_HUNG_TASK_BLOCKER is enabled, the
>>> result is different.
>>
>> The actual blocker tracking for rwsems is only turned on in patch #3.
>> So, there's no case where the feature is active without the cleanup
>> logic already being in place.
>>
>>>
>>>>
>>>> Does this reasoning make sense to you?
>>>
>>> Sorry, no. I think "reader-owner cleanup" is a part of "ownership
>>> tracking" as DEBUG_RWSEMS does (and that keeps consistency of
>>> the ownership tracking behavior same as DEBUG_RWSEM).
>>
>> I thought this step-by-step approach was a bit cleaner, since there are
>> currently only two users for these owner helpers (DEBUG_RWSEMS and
>> DETECT_HUNG_TASK_BLOCKER).
> 
> I think the step-by-step approach fits better if the feature is evolving
> (a working feature is already there.) I don't like the intermediate

Agreed.

> state which does not work correctly, because if we have a unit test(
> like kUnit) it should fail. If you can say "this finds the rwsem

Ah, I missed that ...

> owner as same as what the CONFIG_DEBUG_RWSEM is doing", it is simpler
> to explain what you are doing, and easy to understand.

Thanks for the lesson! Will rework the series as you suggested ;)
Lance