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

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Message-ID: <a8e1f5b5-90a2-4738-821b-afce9ca59df8@linux.dev>
Date: Tue, 24 Jun 2025 13:02:31 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: akpm@...ux-foundation.org, zi.li@...ux.dev, anna.schumaker@...cle.com,
 boqun.feng@...il.com, joel.granados@...nel.org, jstultz@...gle.com,
 kent.overstreet@...ux.dev, leonylgao@...cent.com,
 linux-kernel@...r.kernel.org, longman@...hat.com, mingo@...hat.com,
 mingzhe.yang@...com, peterz@...radead.org, rostedt@...dmis.org,
 senozhatsky@...omium.org, tfiga@...omium.org, will@...nel.org,
 Lance Yang <ioworker0@...il.com>
Subject: Re: [PATCH RFC 2/3] locking/rwsem: clear reader-owner on unlock to
 reduce false positives



On 2025/6/24 11:53, Masami Hiramatsu (Google) wrote:
> On Tue, 24 Jun 2025 09:44:55 +0800
> Lance Yang <lance.yang@...ux.dev> wrote:
> 
>>
>>
>> On 2025/6/24 08:26, Masami Hiramatsu (Google) wrote:
>>> On Thu, 12 Jun 2025 12:19:25 +0800
>>> Lance Yang <ioworker0@...il.com> wrote:
>>>
>>>> From: Lance Yang <lance.yang@...ux.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).

Anyway, if you still feel strongly that they should be merged, I'm happy
to rework the series as you suggested ;p

Thanks,
Lance

> 
> Thank you,
> 
>>
>> Thanks,
>> Lance
>>
>>>
>>> Thanks,
>>>
>>>>
>>>> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
>>>> ---
>>>>    kernel/locking/rwsem.c | 10 ++++------
>>>>    1 file changed, 4 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
>>>> index 6cb29442d4fc..a310eb9896de 100644
>>>> --- a/kernel/locking/rwsem.c
>>>> +++ b/kernel/locking/rwsem.c
>>>> @@ -205,14 +205,12 @@ bool is_rwsem_reader_owned(struct rw_semaphore *sem)
>>>>    		return false;
>>>>    	return rwsem_test_oflags(sem, RWSEM_READER_OWNED);
>>>>    }
>>>> -#endif
>>>>    
>>>> -#ifdef CONFIG_DEBUG_RWSEMS
>>>>    /*
>>>> - * With CONFIG_DEBUG_RWSEMS configured, it will make sure that if there
>>>> - * is a task pointer in owner of a reader-owned rwsem, it will be the
>>>> - * real owner or one of the real owners. The only exception is when the
>>>> - * unlock is done by up_read_non_owner().
>>>> + * With CONFIG_DEBUG_RWSEMS or CONFIG_DETECT_HUNG_TASK_BLOCKER configured,
>>>> + * it will make sure that the owner field of a reader-owned rwsem either
>>>> + * points to a real reader-owner(s) or gets cleared. The only exception is
>>>> + * when the unlock is done by up_read_non_owner().
>>>>     */
>>>>    static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
>>>>    {
>>>> -- 
>>>> 2.49.0
>>>>
>>>
>>>
>>
> 
>