linux-kernel - Re: [PATCH tip/core/rcu 1/3] rcu: Provide polling interfaces for Tree RCU grace periods

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Date:   Mon, 15 Mar 2021 16:11:39 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org
Subject: Re: [PATCH tip/core/rcu 1/3] rcu: Provide polling interfaces for
 Tree RCU grace periods

On Fri, Mar 12, 2021 at 01:26:47PM +0100, Frederic Weisbecker wrote:
> On Wed, Mar 03, 2021 at 04:26:30PM -0800, paulmck@...nel.org wrote:
> >  /**
> > + * start_poll_state_synchronize_rcu - Snapshot and start RCU grace period
> > + *
> > + * Returns a cookie that is used by a later call to cond_synchronize_rcu()
> > + * or poll_state_synchronize_rcu() to determine whether or not a full
> > + * grace period has elapsed in the meantime.  If the needed grace period
> > + * is not already slated to start, notifies RCU core of the need for that
> > + * grace period.
> > + *
> > + * Interrupts must be enabled for the case where it is necessary to awaken
> > + * the grace-period kthread.
> > + */
> > +unsigned long start_poll_synchronize_rcu(void)
> > +{
> > +	unsigned long flags;
> > +	unsigned long gp_seq = get_state_synchronize_rcu();
> 
> Ah! It's using rcu_seq_snap() and not rcu_seq_current() and therefore it's
> waiting for a safe future grace period, right?

Exactly!  ;-)

							Thanx, Paul

> If so, please discard my previous email.
> 
> Thanks.