netdev - Re: [PATCH net-next v4 14/18] sfc: Remove usage of list iterator for list

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Date:   Tue, 19 Apr 2022 08:45:13 +0100
From:   Martin Habets <habetsm.xilinx@...il.com>
To:     Jakob Koschel <jakobkoschel@...il.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        Lars Povlsen <lars.povlsen@...rochip.com>,
        Steen Hegelund <Steen.Hegelund@...rochip.com>,
        UNGLinuxDriver@...rochip.com, Ariel Elior <aelior@...vell.com>,
        Manish Chopra <manishc@...vell.com>,
        Edward Cree <ecree.xilinx@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Jiri Pirko <jiri@...nulli.us>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Casper Andersson <casper.casan@...il.com>,
        Bjarni Jonasson <bjarni.jonasson@...rochip.com>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Arnd Bergmann <arnd@...db.de>,
        Colin Ian King <colin.king@...el.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Xu Wang <vulab@...as.ac.cn>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, bpf@...r.kernel.org,
        Mike Rapoport <rppt@...nel.org>,
        Brian Johannesmeyer <bjohannesmeyer@...il.com>,
        Cristiano Giuffrida <c.giuffrida@...nl>,
        "Bos, H.J." <h.j.bos@...nl>
Subject: Re: [PATCH net-next v4 14/18] sfc: Remove usage of list iterator for
 list_add() after the loop body

On Fri, Apr 15, 2022 at 02:29:43PM +0200, Jakob Koschel wrote:
> In preparation to limit the scope of a list iterator to the list
> traversal loop, use a dedicated pointer pointing to the location
> where the element should be inserted [1].
> 
> Before, the code implicitly used the head when no element was found
> when using &new->list. The new 'pos' variable is set to the list head
> by default and overwritten if the list exits early, marking the
> insertion point for list_add().
> 
> 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/ethernet/sfc/rx_common.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/sfc/rx_common.c b/drivers/net/ethernet/sfc/rx_common.c
> index 1b22c7be0088..716847ba7038 100644
> --- a/drivers/net/ethernet/sfc/rx_common.c
> +++ b/drivers/net/ethernet/sfc/rx_common.c
> @@ -556,6 +556,7 @@ efx_rx_packet_gro(struct efx_channel *channel, struct efx_rx_buffer *rx_buf,
>  struct efx_rss_context *efx_alloc_rss_context_entry(struct efx_nic *efx)
>  {
>  	struct list_head *head = &efx->rss_context.list;
> +	struct list_head *pos = head;

This violates the reverse Xmas list policy. This definition should be
1 line further down.

Martin

>  	struct efx_rss_context *ctx, *new;
>  	u32 id = 1; /* Don't use zero, that refers to the master RSS context */
>  
> @@ -563,8 +564,10 @@ struct efx_rss_context *efx_alloc_rss_context_entry(struct efx_nic *efx)
>  
>  	/* Search for first gap in the numbering */
>  	list_for_each_entry(ctx, head, list) {
> -		if (ctx->user_id != id)
> +		if (ctx->user_id != id) {
> +			pos = &ctx->list;
>  			break;
> +		}
>  		id++;
>  		/* Check for wrap.  If this happens, we have nearly 2^32
>  		 * allocated RSS contexts, which seems unlikely.
> @@ -582,7 +585,7 @@ struct efx_rss_context *efx_alloc_rss_context_entry(struct efx_nic *efx)
>  
>  	/* Insert the new entry into the gap */
>  	new->user_id = id;
> -	list_add_tail(&new->list, &ctx->list);
> +	list_add_tail(&new->list, pos);
>  	return new;
>  }
>  
> -- 
> 2.25.1