linux-kernel - Re: [PATCH rcu 04/12] rcu: Switch polled grace-period APIs to ->gp_seq

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Date:   Wed, 20 Jul 2022 22:49:26 -0700
From:   Boqun Feng <boqun.feng@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, rostedt@...dmis.org,
        Brian Foster <bfoster@...hat.com>,
        Dave Chinner <david@...morbit.com>,
        Al Viro <viro@...iv.linux.org.uk>, Ian Kent <raven@...maw.net>
Subject: Re: [PATCH rcu 04/12] rcu: Switch polled grace-period APIs to
 ->gp_seq_polled

On Wed, Jul 20, 2022 at 09:47:08PM -0700, Paul E. McKenney wrote:
> On Wed, Jul 20, 2022 at 06:51:45PM -0700, Boqun Feng wrote:
> > On Wed, Jul 20, 2022 at 06:04:55PM -0700, Paul E. McKenney wrote:
> > [...]
> > > > > @@ -3860,7 +3944,7 @@ unsigned long get_state_synchronize_rcu(void)
> > > > >  	 * before the load from ->gp_seq.
> > > > >  	 */
> > > > >  	smp_mb();  /* ^^^ */
> > > > > -	return rcu_seq_snap(&rcu_state.gp_seq);
> > > > > +	return rcu_seq_snap(&rcu_state.gp_seq_polled);
> > > > 
> > > > I happened to run into this. There is one usage of
> > > > get_state_synchronize_rcu() in start_poll_synchronize_rcu(), in which
> > > > the return value of get_state_synchronize_rcu() ("gp_seq") will be used
> > > > for rcu_start_this_gp(). I don't think this is quite right, because
> > > > after this change, rcu_state.gp_seq and rcu_state.gp_seq_polled are
> > > > different values, in fact ->gp_seq_polled is greater than ->gp_seq
> > > > by how many synchronize_rcu() is called in early boot.
> > > > 
> > > > Am I missing something here?
> > > 
> > > It does not appear that your are missing anything, sad to say!
> > > 
> > > Does the following make it work better?
> > > 
> > > 							Thanx, Paul
> > > 
> > > ------------------------------------------------------------------------
> > > 
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index 2122359f0c862..cf2fd58a93a41 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -3571,7 +3571,7 @@ EXPORT_SYMBOL_GPL(get_state_synchronize_rcu);
> > >  unsigned long start_poll_synchronize_rcu(void)
> > >  {
> > >  	unsigned long flags;
> > > -	unsigned long gp_seq = get_state_synchronize_rcu();
> > > +	unsigned long gp_seq = rcu_seq_snap(&rcu_state.gp_seq);
> > 
> > get_state_synchronize_rcu() is still needed, because we need to return
> > a cookie for polling for this function. Something like below maybe? Hope
> > I didn't mess up the ordering ;-)
> 
> My thought is to combine your comment with my functionally equivalent
> code that avoids the extra variable.  If that works for you (and if it
> works, for that matter), does Co-developed-by work for you?
> 

Sure! Thanks ;-)

Regards,
Boqun

> 							Thanx, Paul
> 
> > Regards,
> > Boqun
> > 
> > ---------------
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 84d281776688..0f9134871289 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3571,11 +3583,39 @@ EXPORT_SYMBOL_GPL(get_state_synchronize_rcu);
> >  unsigned long start_poll_synchronize_rcu(void)
> >  {
> >         unsigned long flags;
> > -       unsigned long gp_seq = get_state_synchronize_rcu();
> > +       unsigned long gp_seq_poll = get_state_synchronize_rcu();
> > +       unsigned long gp_seq;
> >         bool needwake;
> >         struct rcu_data *rdp;
> >         struct rcu_node *rnp;
> > 
> > +       /*
> > +        * Need to start a gp if no gp has been started yet.
> > +        *
> > +        * Note that we need to snapshot gp_seq after gp_seq_poll, otherwise
> > +        * consider the follow case:
> > +        *
> > +        *      <no gp in progress>     // gp# is 0
> > +        *      snapshot gp_seq         // gp #2 will be set as needed
> > +        *      <a gp passed>
> > +        *                              // gp# is 1
> > +        *      snapshot gp_seq_poll    // polling gets ready until gp #3
> > +        *
> > +        * then the following rcu_start_this_gp() won't mark gp #3 as needed,
> > +        * and polling won't become ready if others don't start a gp.
> > +        *
> > +        * And the following case is fine:
> > +        *
> > +        *      <no gp in progress>     // gp# is 0
> > +        *      snapshot gp_seq_poll    // polling gets ready until gp #2
> > +        *      <a gp passed>
> > +        *                              // gp# is 1
> > +        *      snapshot gp_seq         // gp #3 will be set as needed
> > +        *
> > +        * Also note, we rely on the smp_mb() in get_state_synchronize_rcu()
> > +        * to order the two snapshots.
> > +        */
> > +       gp_seq = rcu_seq_snap(&rcu_state.gp_seq);
> >         lockdep_assert_irqs_enabled();
> >         local_irq_save(flags);
> >         rdp = this_cpu_ptr(&rcu_data);
> > @@ -3585,7 +3625,7 @@ unsigned long start_poll_synchronize_rcu(void)
> >         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> >         if (needwake)
> >                 rcu_gp_kthread_wake();
> > -       return gp_seq;
> > +       return gp_seq_poll;
> >  }
> >  EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu);