On 4/12/21 2:38 AM, Matteo Croce wrote:
> From: Matteo Croce <mcr...@microsoft.com>
>
> 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++)
