Message-ID: <fae2008b-2cca-b483-6b40-d4b95da007d3@redhat.com>
Date:   Thu, 11 Nov 2021 15:45:30 -0500
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.2.0
Subject: Re: [BUG]locking/rwsem: only clean RWSEM_FLAG_HANDOFF when already
 set
Content-Language: en-US
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: <4fafad133b074f279dbab1aa3642e23f@xiaomi.com>
 <20211107090131.1535-1-hdanton@sina.com>
 <13d683ed-793c-b502-44ff-f28114d9386b@redhat.com>
 <02e118c0-2116-b806-2b48-b9c91dc847dd@redhat.com>
 <20211110213854.GE174703@worktop.programming.kicks-ass.net>
 <YY0x55wxO2v5HCOW@hirez.programming.kicks-ass.net>
 <61735528-141c-8d77-592d-b6b8fb75ebaa@redhat.com>
 <YY1s6v9b/tYtNnGv@hirez.programming.kicks-ass.net>
 <e16f9fc2-ce01-192b-065d-460c2ad9b317@redhat.com>
 <20211111203500.GI174703@worktop.programming.kicks-ass.net>
From:   Waiman Long <longman@redhat.com>
In-Reply-To: <20211111203500.GI174703@worktop.programming.kicks-ass.net>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk


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.

Anyway, it will be a follow-up patch.

Cheers,
Longman