Message-ID: <18ff812e-4ccb-479e-b13e-686c40ccb872@redhat.com>
Date:   Thu, 11 Nov 2021 16:54:16 -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>
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 <fae2008b-2cca-b483-6b40-d4b95da007d3@redhat.com>
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From:   Waiman Long <longman@redhat.com>
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On 11/11/21 16:27, Peter Zijlstra wrote:
> On Thu, Nov 11, 2021 at 03:45:30PM -0500, Waiman Long wrote:
>> On 11/11/21 15:35, Peter Zijlstra wrote:
>>> On Thu, Nov 11, 2021 at 02:36:52PM -0500, Waiman Long wrote:
>>>> On 11/11/21 14:20, Peter Zijlstra wrote:
>>>>> On Thu, Nov 11, 2021 at 02:14:48PM -0500, Waiman Long wrote:
>>>>>> As for the PHASE_CHANGE name, we have to be consistent in both rwsem and
>>>>>> mutex. Maybe a follow up patch if you think we should change the
>>>>>> terminology.
>>>>> Well, that's exactly the point, they do radically different things.
>>>>> Having the same name for two different things is confusing.
>>>>>
>>>>> Anyway, let me go read that patch you sent.
>>>> My understanding of handoff is to disable optimistic spinning to let waiters
>>>> in the wait queue have an opportunity to acquire the lock. There are
>>>> difference in details on how to do that in mutex and rwsem, though.
>>> Ah, but the mutex does an actual hand-off, it hands the lock to a
>>> specific waiting task. That is, unlock() sets owner, as opposed to
>>> trylock().
>>>
>>> The rwsem code doesn't, it just forces a phase change. Once a waiter has
>>> been blocked too long, the handoff bit is set, causing new readers to be
>>> blocked. Then we wait for existing readers to complete. At that point,
>>> any next waiter (most likely a writer) should really get the lock (and
>>> in that regards the rwsem code is a bit funny).
>>>
>>> So while both ensure fairness, the means of doing so is quite different.
>>> One hands the lock ownership to a specific waiter, the other arranges
>>> for a quiescent state such that the next waiter can proceed.
>> That is a valid argument. However, the name PHASE_CHANGE sounds weird to me.
>> I am not objecting to changing the term, but probably with a better name
>> NO_OPTSPIN, NO_LOCKSTEALING or something like that to emphasize that fact
>> that optimistic spinning or lock stealing should not be allowed.
> RWSEM_FLAG_QUIESCE ?

I think that is a more acceptable term than PHASE_CHANGE. Will have a 
follow up patch later on. This one is more urgent and I want to get it 
done first.

Cheers,
Longman.