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

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Message-ID: <20210316164229.GV2696@paulmck-ThinkPad-P72>
Date:   Tue, 16 Mar 2021 09:42:29 -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 Tue, Mar 16, 2021 at 03:47:22PM +0100, Frederic Weisbecker wrote:
> On Wed, Mar 03, 2021 at 04:26:30PM -0800, paulmck@...nel.org wrote:
> > +/**
> > + * poll_state_synchronize_rcu - Conditionally wait for an RCU grace period
> > + *
> > + * @oldstate: return from call to get_state_synchronize_rcu() or start_poll_synchronize_rcu()
> > + *
> > + * If a full RCU grace period has elapsed since the earlier call from
> > + * which oldstate was obtained, return @true, otherwise return @false.
> > + * Otherwise, invoke synchronize_rcu() to wait for a full grace period.
> > + *
> > + * Yes, this function does not take counter wrap into account.
> > + * But counter wrap is harmless.  If the counter wraps, we have waited for
> > + * more than 2 billion grace periods (and way more on a 64-bit system!).
> > + * Those needing to keep oldstate values for very long time periods
> > + * (many hours even on 32-bit systems) should check them occasionally
> > + * and either refresh them or set a flag indicating that the grace period
> > + * has completed.
> > + */
> > +bool poll_state_synchronize_rcu(unsigned long oldstate)
> > +{
> > +	if (rcu_seq_done(&rcu_state.gp_seq, oldstate)) {
> > +		smp_mb(); /* Ensure GP ends before subsequent accesses. */
> > +		return true;
> > +	}
> > +	return false;
> > +}
> > +EXPORT_SYMBOL_GPL(poll_state_synchronize_rcu);
> > +
> > +/**
> >   * cond_synchronize_rcu - Conditionally wait for an RCU grace period
> >   *
> >   * @oldstate: return value from earlier call to get_state_synchronize_rcu()
> >   *
> >   * If a full RCU grace period has elapsed since the earlier call to
> > - * get_state_synchronize_rcu(), just return.  Otherwise, invoke
> > - * synchronize_rcu() to wait for a full grace period.
> > + * get_state_synchronize_rcu() or start_poll_synchronize_rcu(), just return.
> > + * Otherwise, invoke synchronize_rcu() to wait for a full grace period.
> >   *
> >   * Yes, this function does not take counter wrap into account.  But
> >   * counter wrap is harmless.  If the counter wraps, we have waited for
> > @@ -3804,7 +3864,7 @@ EXPORT_SYMBOL_GPL(get_state_synchronize_rcu);
> >   */
> >  void cond_synchronize_rcu(unsigned long oldstate)
> >  {
> > -	if (!rcu_seq_done(&rcu_state.gp_seq, oldstate))
> > +	if (!poll_state_synchronize_rcu(oldstate))
> >  		synchronize_rcu();
> >  	else
> >  		smp_mb(); /* Ensure GP ends before subsequent accesses. */
> 
> poll_state_synchronize_rcu() already does the full barrier.

Right you are!  Good catch, thank you!!!

I will fold removing this smp_mb() in with attribution.

							Thanx, Paul