linux-kernel - Re: [PATCH 1/2] rcu: Do not release a wait-head from a GP kthread

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Message-ID: <CAEXW_YTzMPxdm=UmMScppfP2WTqoyo6Z4c0p06HNgT-NAL2ciQ@mail.gmail.com>
Date: Wed, 6 Mar 2024 17:44:04 -0500
From: Joel Fernandes <joel@...lfernandes.org>
To: "Uladzislau Rezki (Sony)" <urezki@...il.com>, "Paul E . McKenney" <paulmck@...nel.org>
Cc: RCU <rcu@...r.kernel.org>, Neeraj upadhyay <Neeraj.Upadhyay@....com>, 
	Boqun Feng <boqun.feng@...il.com>, Hillf Danton <hdanton@...a.com>, 
	LKML <linux-kernel@...r.kernel.org>, 
	Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>, Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH 1/2] rcu: Do not release a wait-head from a GP kthread

On Wed, Mar 6, 2024 at 5:31 PM Joel Fernandes <joel@...lfernandes.org> wrote:
>
>
>
> On 3/5/2024 2:57 PM, Uladzislau Rezki (Sony) wrote:
> > Fix a below race by not releasing a wait-head from the
> > GP-kthread as it can lead for reusing it whereas a worker
> > can still access it thus execute newly added callbacks too
> > early.
> >
[...]
> There might a way to prevent queuing new work as fast-path optimization, incase
> the CBs per GP will always be < SR_MAX_USERS_WAKE_FROM_GP but I could not find a
> workqueue API that helps there, and work_busy() has comments saying not to use that.

One way to do this would be to maintain a count of how many CBs are in
flight via the worker route, and then fast-path-free the thing if the
count is 0. Should I send a patch around something like that? It saves
1 more wakeup per synchronize_rcu() I think.

 - Joel