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

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Message-ID: <20220721044708.GU1790663@paulmck-ThinkPad-P17-Gen-1>
Date:   Wed, 20 Jul 2022 21:47:08 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Boqun Feng <boqun.feng@...il.com>
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 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?

							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);