Message-ID: <1158f843-ee83-8dea-fade-e59f529a88fb@redhat.com>
Date:   Thu, 11 Nov 2021 16:55:12 -0500
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Subject: Re: [BUG]locking/rwsem: only clean RWSEM_FLAG_HANDOFF when already
 set
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To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Hillf Danton <hdanton@sina.com>,
        =?UTF-8?B?6ams5oyv5Y2O?= <mazhenhua@xiaomi.com>,
        mingo <mingo@redhat.com>, will <will@kernel.org>,
        "boqun.feng" <boqun.feng@gmail.com>,
        linux-kernel <linux-kernel@vger.kernel.org>
References: <13d683ed-793c-b502-44ff-f28114d9386b@redhat.com>
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 <61735528-141c-8d77-592d-b6b8fb75ebaa@redhat.com>
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 <e16f9fc2-ce01-192b-065d-460c2ad9b317@redhat.com>
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 <436bcf39-297a-f5a6-ac58-a82e77cb3b83@redhat.com>
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From:   Waiman Long <longman@redhat.com>
In-Reply-To: <YY2Q220iEiFdaKlT@hirez.programming.kicks-ass.net>
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On 11/11/21 16:53, Peter Zijlstra wrote:
> On Thu, Nov 11, 2021 at 04:25:56PM -0500, Waiman Long wrote:
>> On 11/11/21 16:01, Waiman Long wrote:
>>> On 11/11/21 15:26, Peter Zijlstra wrote:
>>>> On Thu, Nov 11, 2021 at 02:36:52PM -0500, Waiman Long wrote:
>>>>
>>>>> @@ -434,6 +430,7 @@ static void rwsem_mark_wake(struct
>>>>> rw_semaphore *sem,
>>>>>                if (!(oldcount & RWSEM_FLAG_HANDOFF) &&
>>>>>                    time_after(jiffies, waiter->timeout)) {
>>>>>                    adjustment -= RWSEM_FLAG_HANDOFF;
>>>>> +                waiter->handoff_set = true;
>>>>>                    lockevent_inc(rwsem_rlock_handoff);
>>>>>                }
>>>> Do we really need this flag? Wouldn't it be the same as waiter-is-first
>>>> AND sem-has-handoff ?
>>> That is true. The only downside is that we have to read the count first
>>> in rwsem_out_nolock_clear_flags(). Since this is not a fast path, it
>>> should be OK to do that.
>> I just realize that I may still need this flag for writer to determine if it
>> should spin after failing to acquire the lock. Or I will have to do extra
>> read of count value in the loop. I don't need to use it for writer now.
> Maybe it's too late here, but afaict this is right after failing
> try_write_lock(), which will have done at least that load you're
> interested in, no?
>
> Simply have try_write_lock() update &count or something.

You are right. I have actually decided to do an extra read after second 
thought.

Cheers,
Longman