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

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Message-ID: <ZemyHxqjbiUrb9kf@pc636>
Date: Thu, 7 Mar 2024 13:25:03 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: "Uladzislau Rezki (Sony)" <urezki@...il.com>,
	"Paul E . McKenney" <paulmck@...nel.org>, 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 06, 2024 at 05:44:04PM -0500, Joel Fernandes wrote:
> 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.
> 
We can release the last wait-head if we know that the worker is not
pending/running. Then we guarantee that Frederic's case is not possible.
>From the other hand it will introduce again more mess because the idea
was, in the begging, to start with something really simple :)

--
Uladzislau Rezki