From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <longman@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
linux-kernel@vger.kernel.org, Davidlohr Bueso <dave@stgolabs.net>,
mazhenhua <mazhenhua@xiaomi.com>, Hillf Danton <hdanton@sina.com>,
Maria Yu <quic_aiquny@quicinc.com>
Subject: Re: [PATCH v5] locking/rwsem: Make handoff bit handling more consistent
Date: Wed, 17 Nov 2021 14:36:29 +0100 [thread overview]
Message-ID: <YZUFXeqgqJwEuDdG@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20211116012912.723980-1-longman@redhat.com>
On Mon, Nov 15, 2021 at 08:29:12PM -0500, Waiman Long wrote:
> There are some inconsistency in the way that the handoff bit is being
> handled in readers and writers.
>
> Firstly, when a queue head writer set the handoff bit, it will clear it
> when the writer is being killed or interrupted on its way out without
> acquiring the lock. That is not the case for a queue head reader. The
> handoff bit will simply be inherited by the next waiter.
>
> Secondly, in the out_nolock path of rwsem_down_read_slowpath(), both
> the waiter and handoff bits are cleared if the wait queue becomes empty.
> For rwsem_down_write_slowpath(), however, the handoff bit is not checked
> and cleared if the wait queue is empty. This can potentially make the
> handoff bit set with empty wait queue.
>
> To make the handoff bit handling more consistent and robust, extract
> out handoff bit clearing code into the new rwsem_del_waiter() helper
> function. The common function will only use atomic_long_andnot() to
> clear bits when the wait queue is empty to avoid possible race condition.
> If the first waiter with handoff bit set is killed or interrupted to
> exit the slowpath without acquiring the lock, the next waiter will
> inherit the handoff bit.
>
> While at it, simplify the trylock for loop in rwsem_down_write_slowpath()
> to make it easier to read.
>
> Fixes: 4f23dbc1e657 ("locking/rwsem: Implement lock handoff to prevent lock starvation")
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
Zhenhua Ma, would you be able to confirm this works for you and provide
a Tested-by?
next prev parent reply other threads:[~2021-11-17 13:36 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-16 1:29 [PATCH v5] locking/rwsem: Make handoff bit handling more consistent Waiman Long
2021-11-16 2:52 ` Aiqun(Maria) Yu
2021-11-16 9:14 ` Peter Zijlstra
2021-11-16 9:24 ` Peter Zijlstra
2021-11-16 14:52 ` Waiman Long
2021-11-17 13:36 ` Peter Zijlstra [this message]
2021-11-23 8:53 ` [tip: locking/urgent] " tip-bot2 for Waiman Long
2022-02-14 15:47 ` Re:[PATCH v5] " chenguanyou
2022-02-14 16:01 ` [PATCH " Greg KH
2022-04-11 18:26 ` john.p.donnelly
2022-04-11 18:40 ` Waiman Long
2022-04-11 21:03 ` john.p.donnelly
2022-04-11 21:07 ` Waiman Long
2022-04-12 16:28 ` john.p.donnelly
2022-04-12 17:04 ` Waiman Long
2022-04-14 10:48 ` Greg KH
2022-04-14 15:18 ` Waiman Long
2022-04-14 15:42 ` Greg KH
2022-04-14 15:44 ` Waiman Long
2022-04-20 13:55 ` john.p.donnelly
2022-04-26 20:21 ` Waiman Long
2022-04-26 21:22 ` john.p.donnelly
2022-02-14 16:22 ` chenguanyou
2022-02-15 7:41 ` [PATCH " Greg KH
2022-02-16 16:30 ` Waiman Long
2022-02-17 15:41 ` chenguanyou
2022-03-14 8:07 ` [PATCH " Greg KH
2022-03-22 2:49 ` chenguanyou
2022-03-24 12:51 ` [PATCH " Greg KH
2022-07-19 0:27 ` Doug Anderson
[not found] ` <20220719104104.1634-1-hdanton@sina.com>
2022-07-19 15:30 ` Doug Anderson
[not found] ` <20220722115510.2101-1-hdanton@sina.com>
2022-07-22 14:02 ` Doug Anderson
[not found] ` <20220723001713.2156-1-hdanton@sina.com>
2022-08-05 17:14 ` Doug Anderson
2022-08-05 19:02 ` Waiman Long
2022-08-05 19:16 ` Doug Anderson
2022-08-30 16:18 ` Doug Anderson
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