linux-kernel - Re: [PATCH v2 rcu 05/16] rcu: Refactor code a bit in rcu_nocb_do_flush

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Date:   Wed, 23 Nov 2022 09:54:37 -0800
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@...a.com, rostedt@...dmis.org,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>
Subject: Re: [PATCH v2 rcu 05/16] rcu: Refactor code a bit in
 rcu_nocb_do_flush_bypass()

On Wed, Nov 23, 2022 at 04:59:29PM +0100, Frederic Weisbecker wrote:
> On Mon, Nov 21, 2022 at 05:04:10PM -0800, Paul E. McKenney wrote:
> > From: "Joel Fernandes (Google)" <joel@...lfernandes.org>
> > 
> > This consolidates the code a bit and makes it cleaner. Functionally it
> > is the same.
> > 
> > Reported-by: Paul E. McKenney <paulmck@...nel.org>
> > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > ---
> >  kernel/rcu/tree_nocb.h | 17 +++++++++--------
> >  1 file changed, 9 insertions(+), 8 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > index d6e4c076b0515..213daf81c057f 100644
> > --- a/kernel/rcu/tree_nocb.h
> > +++ b/kernel/rcu/tree_nocb.h
> > @@ -327,10 +327,11 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
> >   *
> >   * Note that this function always returns true if rhp is NULL.
> >   */
> > -static bool rcu_nocb_do_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> > +static bool rcu_nocb_do_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp_in,
> >  				     unsigned long j, bool lazy)
> >  {
> >  	struct rcu_cblist rcl;
> > +	struct rcu_head *rhp = rhp_in;
> 
> Why that intermediate rhp_in?

To avoid modifying the formal parameter, should the original value prove
useful, for example, for tracing or debugging.

> >  	WARN_ON_ONCE(!rcu_rdp_is_offloaded(rdp));
> >  	rcu_lockdep_assert_cblist_protected(rdp);
> > @@ -345,16 +346,16 @@ static bool rcu_nocb_do_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> >  
> >  	/*
> >  	 * If the new CB requested was a lazy one, queue it onto the main
> > -	 * ->cblist so we can take advantage of a sooner grade period.
> > +	 * ->cblist so that we can take advantage of the grace-period that will
> > +	 * happen regardless. But queue it onto the bypass list first so that
> > +	 * the lazy CB is ordered with the existing CBs in the bypass list.
> >  	 */
> >  	if (lazy && rhp) {
> > -		rcu_cblist_flush_enqueue(&rcl, &rdp->nocb_bypass, NULL);
> > -		rcu_cblist_enqueue(&rcl, rhp);
> > -		WRITE_ONCE(rdp->lazy_len, 0);
> > -	} else {
> > -		rcu_cblist_flush_enqueue(&rcl, &rdp->nocb_bypass, rhp);
> > -		WRITE_ONCE(rdp->lazy_len, 0);
> > +		rcu_cblist_enqueue(&rdp->nocb_bypass, rhp);
> > +		rhp = NULL;
> >  	}
> > +	rcu_cblist_flush_enqueue(&rcl, &rdp->nocb_bypass, rhp);
> > +	WRITE_ONCE(rdp->lazy_len, 0);
> 
> Reviewed-by: Frederic Weisbecker <frederic@...nel.org>

Thank you!  I will apply this on my next rebase.

							Thanx, Paul

> Thanks.
> 
> >  
> >  	rcu_segcblist_insert_pend_cbs(&rdp->cblist, &rcl);
> >  	WRITE_ONCE(rdp->nocb_bypass_first, j);
> > -- 
> > 2.31.1.189.g2e36527f23
> >