linux-kernel - Re: [BUG]locking/rwsem: only clean RWSEM_FLAG

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Date:   Thu, 11 Nov 2021 14:52:19 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Hillf Danton <hdanton@...a.com>,
        马振华 <mazhenhua@...omi.com>,
        mingo <mingo@...hat.com>, will <will@...nel.org>,
        "boqun.feng" <boqun.feng@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [BUG]locking/rwsem: only clean RWSEM_FLAG_HANDOFF when already
 set


On 11/11/21 14:36, 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.
>
> BTW, I have decided that we should further simply the trylock for loop 
> in rwsem_down_write_slowpath() to make it easier to read. That is the 
> only difference in the attached v2 patch compared with the previous one.

My bad, I forgot to initialize waiter.handoff_set in 
rwsem_down_write_slowpath(). I will send out an updated version once you 
have finished your review.

Cheers,
Longman