From: Waiman Long <longman@redhat.com>
To: Hillf Danton <hdanton@sina.com>
Cc: "Mukesh Ojha" <quic_mojha@quicinc.com>,
"Peter Zijlstra" <peterz@infradead.org>,
"Ingo Molnar" <mingo@redhat.com>, "Will Deacon" <will@kernel.org>,
"Boqun Feng" <boqun.feng@gmail.com>,
linux-kernel@vger.kernel.org, john.p.donnelly@oracle.com,
"Ting11 Wang 王婷" <wangting11@xiaomi.com>
Subject: Re: [PATCH v3 4/5] locking/rwsem: Enable direct rwsem lock handoff
Date: Wed, 19 Oct 2022 11:02:00 -0400 [thread overview]
Message-ID: <28264558-5f3f-9521-ced2-2c94a27002ec@redhat.com> (raw)
In-Reply-To: <20221019070559.1220-1-hdanton@sina.com>
On 10/19/22 03:05, Hillf Danton wrote:
> On 18 Oct 2022 22:49:02 -0400 Waiman Long <longman@redhat.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
next prev parent reply other threads:[~2022-10-19 15:10 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-17 21:13 [PATCH v3 0/5] lockinig/rwsem: Fix rwsem bugs & enable true lock handoff Waiman Long
2022-10-17 21:13 ` [PATCH v3 1/5] locking/rwsem: Prevent non-first waiter from spinning in down_write() slowpath Waiman Long
2022-10-24 13:17 ` Peter Zijlstra
2022-10-24 13:50 ` Waiman Long
2022-10-17 21:13 ` [PATCH v3 2/5] locking/rwsem: Limit # of null owner retries for handoff writer Waiman Long
2022-10-24 13:31 ` Peter Zijlstra
2022-10-24 15:55 ` Waiman Long
2022-10-25 11:22 ` Peter Zijlstra
2022-10-25 11:48 ` Peter Zijlstra
2022-10-25 19:55 ` Waiman Long
2022-10-25 20:14 ` Peter Zijlstra
2022-10-26 1:44 ` Waiman Long
[not found] ` <20221025145843.2953-1-hdanton@sina.com>
2022-10-25 19:00 ` Waiman Long
2022-10-17 21:13 ` [PATCH v3 3/5] locking/rwsem: Change waiter->hanodff_set to a handoff_state enum Waiman Long
2022-10-17 21:13 ` [PATCH v3 4/5] locking/rwsem: Enable direct rwsem lock handoff Waiman Long
2022-10-17 21:13 ` [PATCH v3 5/5] locking/rwsem: Update handoff lock events tracking Waiman Long
[not found] ` <20221018111424.1007-1-hdanton@sina.com>
2022-10-18 14:13 ` [PATCH v3 4/5] locking/rwsem: Enable direct rwsem lock handoff Mukesh Ojha
2022-10-18 17:37 ` Waiman Long
[not found] ` <20221018235138.1088-1-hdanton@sina.com>
2022-10-19 0:39 ` Waiman Long
[not found] ` <20221019022934.1166-1-hdanton@sina.com>
2022-10-19 2:49 ` Waiman Long
[not found] ` <20221019070559.1220-1-hdanton@sina.com>
2022-10-19 15:02 ` Waiman Long [this message]
2022-10-24 16:18 ` Waiman Long
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