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

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Message-ID: <28264558-5f3f-9521-ced2-2c94a27002ec@redhat.com>
Date:   Wed, 19 Oct 2022 11:02:00 -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 10/19/22 03:05, Hillf Danton wrote:
> On 18 Oct 2022 22:49:02 -0400 Waiman Long <longman@...hat.com>
>> On 10/18/22 22:29, Hillf Danton wrote:
>>> If handoff grants rwsem to a read waiter then the read fast path may revive.
>> I don't quite understand what you mean by "read fast path may revive".
> Subsequent readers may take rwsem without going the slow path after a
> read waiter is granted.
That may not be true. As long as there are still waiters in the wait 
queue, a reader has to go into the slow path and queued up in the wait 
queue. This is is to prevent a continuous stream of readers from 
starving writers in the wait queue.
>
> OTOH even after the check for single BIAS, another reader may come in
> and add its BIAS to count, which builds the same count as multiple
> readers came in before the check.

That is true, but hopefully we will eventually run out reader trying to 
get a read lock on a given rwsem.

Cheers,
Longman