linux-kernel - Re: [PATCH v5] locking/rwsem: Make handoff bit handling more consistent

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Date:   Tue, 12 Apr 2022 13:04:05 -0400
From:   Waiman Long <longman@...hat.com>
To:     john.p.donnelly@...cle.com,
        chenguanyou <chenguanyou9338@...il.com>,
        gregkh@...uxfoundation.org
Cc:     dave@...olabs.net, hdanton@...a.com, linux-kernel@...r.kernel.org,
        mazhenhua@...omi.com, mingo@...hat.com, peterz@...radead.org,
        quic_aiquny@...cinc.com, will@...nel.org, sashal@...nel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH v5] locking/rwsem: Make handoff bit handling more
 consistent

On 4/12/22 12:28, john.p.donnelly@...cle.com wrote:
> On 4/11/22 4:07 PM, Waiman Long wrote:
>>
>> On 4/11/22 17:03, john.p.donnelly@...cle.com wrote:
>>>
>>>>>
>>>>> I have reached out to Waiman and he suggested this for our next 
>>>>> test pass:
>>>>>
>>>>>
>>>>> 1ee326196c6658 locking/rwsem: Always try to wake waiters in 
>>>>> out_nolock path
>>>>
>>>> Does this commit help to avoid the lockup problem?
>>>>
>>>> Commit 1ee326196c6658 fixes a potential missed wakeup problem when 
>>>> a reader first in the wait queue is interrupted out without 
>>>> acquiring the lock. It is actually not a fix for commit 
>>>> d257cc8cb8d5. However, this commit changes the out_nolock path 
>>>> behavior of writers by leaving the handoff bit set when the wait 
>>>> queue isn't empty. That likely makes the missed wakeup problem 
>>>> easier to reproduce.
>>>>
>>>> Cheers,
>>>> Longman
>>>>
>>>
>>> Hi,
>>>
>>>
>>> We are testing now
>>>
>>> ETA for fio soak test completion is  ~15hr from now.
>>>
>>> I wanted to share the stack traces for future reference + occurrences.
>>>
>> I am looking forward to your testing results tomorrow.
>>
>> Cheers,
>> Longman
>>
> Hi
>
>  Our 24hr fio soak test with :
>
>  1ee326196c6658 locking/rwsem: Always try to wake waiters in 
> out_nolock path
>
>
>  applied to 5.15.30  passed.
>
>  I suggest you append  1ee326196c6658 with :
>
>
>  cc: stable
>
>   Fixes: d257cc8cb8d5 ("locking/rwsem: Make handoff bit handling more 
> consistent")
>
>
> I'll leave the implementation details up to the core maintainers how 
> to do that ;-)

Thanks for the test.

The patch has already been in the tip tree. It may not be easy to add a 
Fixes tag to it. Anyway, I will encourage stable tree maintainer to take 
it as it does fix a problem as shown in your test.

Cheers,
Longman