linux-kernel - Re: [PATCH v3 1/1] rcu: Reduce synchronize

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Message-ID: <ZS-_kS0vr8G-k0pp@pc636>
Date:   Wed, 18 Oct 2023 13:20:49 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Hillf Danton <hdanton@...a.com>
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>,
        LKML <linux-kernel@...r.kernel.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH v3 1/1] rcu: Reduce synchronize_rcu() waiting time

Hello, Hillf!

> Hi Ulad
> 
> Good work with a nit.
> 
Thank you :)

> On Mon, Oct 16, 2023 at 1:30 PM Uladzislau Rezki (Sony) <urezki@...il.com> wrote:
> > +static void rcu_sr_normal_gp_init(void)
> > +{
> > +	struct llist_node *llnode, *rcu;
> > +	int ret;
> > +
> > +	if (llist_empty(&sr.curr))
> > +		return;
> 
> This empty check erases the curr_tail race below instead of
> atomic_inc_return(&sr.active), because llist_add() will never return true
> after this check.
>
I use "active" counter to guarantee that a tail was updated in the
rcu_sr_normal_add_req(), i.e. the list might be not empty whereas the
tail updating might be in progress. llist_add() success and the task gets
preemted as an example.

Or i miss your point? If so, i appreciate if you clarify it in more
detail.

> > +
> > +	/*
> > +	 * A waiting list of GP should be empty on this step,
> > +	 * since a GP-kthread, rcu_gp_init() -> gp_cleanup(),
> > +	 * rolls it over. If not, it is a BUG, warn a user.
> > +	 */
> > +	WARN_ON_ONCE(!llist_empty(&sr.wait));
> > +
> > +	/*
> > +	 * Obtain a tail of current active users. It is guaranteed
> > +	 * that if we are only one active user and the list is not
> > +	 * empty, the tail has already been updated.
> > +	 */
> > +	ret = atomic_inc_return(&sr.active);
> 
> Replacing atomic_inc_return() with smp_mb() cuts sr.active off.
> 
But here we would like to know that we were only one user + not
empty list gurantees that a tail is ready.

Thank you for your comments!

--
Uladzislau Rezki