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

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Message-ID: <20221123155929.GB1387380@lothringen>
Date:   Wed, 23 Nov 2022 16:59:29 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...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 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?

>  
>  	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>

Thanks.

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