Re: [PATCH net-next v2 2/3] net: use skb_for_each_frag() helper where possible

[Eric Dumazet]

On 4/12/21 2:38 AM, Matteo Croce wrote:
> From: Matteo Croce <mcroce@xxxxxxxxxxxxx>
> 
> use the new helper macro skb_for_each_frag() which allows to iterate
> through all the SKB fragments.
> 
> The patch was created with Coccinelle, this was the semantic patch:
> 
> @@
> struct sk_buff *skb;
> identifier i;
> statement S;
> iterator name skb_for_each_frag;
> @@
> -for (i = 0; i < skb_shinfo(skb)->nr_frags; \(++i\|i++\))
> +skb_for_each_frag(skb, i)
>  S
> @@
> struct skb_shared_info *sinfo;
> struct sk_buff *skb;
> identifier i;
> statement S;
> iterator name skb_for_each_frag;
> @@


I disagree with this part :

>  sinfo = skb_shinfo(skb)
>  ...
> -for (i = 0; i < sinfo->nr_frags; \(++i\|i++\))
> +skb_for_each_frag(skb, i)
>  S
>


> index bde781f46b41..5de00477eaf9 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -1644,7 +1644,7 @@ static int __pskb_trim_head(struct sk_buff *skb, int len)
>  	eat = len;
>  	k = 0;
>  	shinfo = skb_shinfo(skb);
> -	for (i = 0; i < shinfo->nr_frags; i++) {
> +	skb_for_each_frag(skb, i) {
>  		int size = skb_frag_size(&shinfo->frags[i]);
>  
>  		if (size <= eat) {

This will force the compiler to re-evaluate skb_shinfo(skb)->nr_frags in the loop,
since atomic operations like skb_frag_unref() have a memory clobber.

skb_shinfo(skb)->nr_frags has to reload three vars.

The macro should only be used when the code had

for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)