linux-kernel - Re: [PATCH v3 4/5] locking/rwsem: Enable direct rwsem lock handoff

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Message-ID: <03113528-65ad-96b6-3580-d88613a246c9@redhat.com>
Date:   Tue, 18 Oct 2022 20:39:59 -0400
From:   Waiman Long <longman@...hat.com>
To:     Hillf Danton <hdanton@...a.com>
Cc:     Mukesh Ojha <quic_mojha@...cinc.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-kernel@...r.kernel.org, john.p.donnelly@...cle.com,
        Ting11 Wang 王婷 <wangting11@...omi.com>
Subject: Re: [PATCH v3 4/5] locking/rwsem: Enable direct rwsem lock handoff

On 10/18/22 19:51, Hillf Danton wrote:
> On 18 Oct 2022 13:37:20 -0400 Waiman Long <longman@...hat.com>
>> On 10/18/22 10:13, Mukesh Ojha wrote:
>>> On 10/18/2022 4:44 PM, Hillf Danton wrote:
>>>> On 17 Oct 2022 17:13:55 -0400 Waiman Long <longman@...hat.com>
>>>>> @@ -1067,13 +1119,33 @@ rwsem_down_read_slowpath(struct rw_semaphore
>>>>>                return sem;
>>>>>            }
>>>>>            adjustment += RWSEM_FLAG_WAITERS;
>>>>> +    } else if ((count & RWSEM_FLAG_HANDOFF) &&
>>>>> +          ((count & RWSEM_LOCK_MASK) == RWSEM_READER_BIAS)) {
>>>> Could a couple of CPUs go read slow path in parallel?
>>>>
>> This is under wait_lock protection. So no parallel execution is possible.
> They individually add RWSEM_READER_BIAS to count before taking wait_lock,
> and the check for BIAS here does not cover the case of readers in parallel.
> Is this intended?
>
> Hillf

As I said in the patch description, the lock handoff can only be done if 
we can be sure that there is no other active locks outstanding with the 
handoff bit set. If at the time of the check, another reader come in and 
adds its RWSEM_READER_BIAS, the check fail and the cpu will proceed to 
put its waiter in the queue and begin sleeping. Hopefully, the last one 
left will find that count has only its RWSEM_READER_BIAS and it can 
start the handoff process.

Cheers,
Longman