netdev - Re: [PATCH net-next 02/15] net: dsa: sja1105: Remove usage of iterator for list

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Message-ID: <cd02715e-890d-bf67-697a-8f9b06160536@csgroup.eu>
Date:   Fri, 8 Apr 2022 07:47:59 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Jakob Koschel <jakobkoschel@...il.com>,
        "David S. Miller" <davem@...emloft.net>
CC:     Andrew Lunn <andrew@...n.ch>,
        Cristiano Giuffrida <c.giuffrida@...nl>,
        Eric Dumazet <edumazet@...gle.com>,
        Paul Mackerras <paulus@...ba.org>,
        Ariel Elior <aelior@...vell.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Manish Chopra <manishc@...vell.com>,
        Steen Hegelund <Steen.Hegelund@...rochip.com>,
        "Bos, H.J." <h.j.bos@...nl>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Martin Habets <habetsm.xilinx@...il.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Bjarni Jonasson <bjarni.jonasson@...rochip.com>,
        Jiri Pirko <jiri@...nulli.us>, Arnd Bergmann <arnd@...db.de>,
        Brian Johannesmeyer <bjohannesmeyer@...il.com>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Jakub Kicinski <kuba@...nel.org>, Di Zhu <zhudi21@...wei.com>,
        Lars Povlsen <lars.povlsen@...rochip.com>,
        Colin Ian King <colin.king@...el.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "UNGLinuxDriver@...rochip.com" <UNGLinuxDriver@...rochip.com>,
        Edward Cree <ecree.xilinx@...il.com>,
        Michael Walle <michael@...le.cc>, Xu Wang <vulab@...as.ac.cn>,
        Vladimir Oltean <olteanv@...il.com>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        Casper Andersson <casper.casan@...il.com>,
        Mike Rapoport <rppt@...nel.org>
Subject: Re: [PATCH net-next 02/15] net: dsa: sja1105: Remove usage of
 iterator for list_add() after loop



Le 07/04/2022 à 12:28, Jakob Koschel a écrit :
> In preparation to limit the scope of a list iterator to the list
> traversal loop, use a dedicated pointer to point to the found element [1].
> 
> Before, the code implicitly used the head when no element was found
> when using &pos->list. Since the new variable is only set if an
> element was found, the list_add() is performed within the loop
> and only done after the loop if it is done on the list head directly.
> 
> Link: https://lore.kernel.org/all/CAHk-=wgRr_D8CB-D9Kg-c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@mail.gmail.com/ [1]
> Signed-off-by: Jakob Koschel <jakobkoschel@...il.com>
> ---
>   drivers/net/dsa/sja1105/sja1105_vl.c | 14 +++++++++-----
>   1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/dsa/sja1105/sja1105_vl.c b/drivers/net/dsa/sja1105/sja1105_vl.c
> index b7e95d60a6e4..cfcae4d19eef 100644
> --- a/drivers/net/dsa/sja1105/sja1105_vl.c
> +++ b/drivers/net/dsa/sja1105/sja1105_vl.c
> @@ -27,20 +27,24 @@ static int sja1105_insert_gate_entry(struct sja1105_gating_config *gating_cfg,
>   	if (list_empty(&gating_cfg->entries)) {
>   		list_add(&e->list, &gating_cfg->entries);
>   	} else {
> -		struct sja1105_gate_entry *p;
> +		struct sja1105_gate_entry *p = NULL, *iter;
>   
> -		list_for_each_entry(p, &gating_cfg->entries, list) {
> -			if (p->interval == e->interval) {
> +		list_for_each_entry(iter, &gating_cfg->entries, list) {
> +			if (iter->interval == e->interval) {
>   				NL_SET_ERR_MSG_MOD(extack,
>   						   "Gate conflict");
>   				rc = -EBUSY;
>   				goto err;
>   			}
>   
> -			if (e->interval < p->interval)
> +			if (e->interval < iter->interval) {
> +				p = iter;
> +				list_add(&e->list, iter->list.prev);
>   				break;
> +			}
>   		}
> -		list_add(&e->list, p->list.prev);
> +		if (!p)
> +			list_add(&e->list, gating_cfg->entries.prev);
>   	}
>   
>   	gating_cfg->num_entries++;

This change looks ugly, why duplicating the list_add() to do the same ? 
At the end of the loop the pointer contains gating_cfg->entries, so it 
was cleaner before.

If you don't want to use the loop index outside the loop, fair enough, 
all you have to do is:

		struct sja1105_gate_entry *p, *iter;

		list_for_each_entry(iter, &gating_cfg->entries, list) {
			if (iter->interval == e->interval) {
				NL_SET_ERR_MSG_MOD(extack,
						   "Gate conflict");
				rc = -EBUSY;
				goto err;
			}
			p = iter;

			if (e->interval < iter->interval)
				break;
		}
		list_add(&e->list, p->list.prev);



Christophe