Re: [PATCH net-next v2 2/3] net: use skb_for_each_frag() helper where possible
flat view
From: Eric Dumazet <hidden>
Date: 2021-04-12 07:40:26
Also in:
lkml
On 4/12/21 2:38 AM, Matteo Croce wrote:
From: Matteo Croce <redacted> 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 :
quoted hunk ↗ jump to hunk
sinfo = skb_shinfo(skb) ... -for (i = 0; i < sinfo->nr_frags; \(++i\|i++\)) +skb_for_each_frag(skb, i) S
quoted hunk ↗ jump to hunk
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++)