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    SubjectRe: [PATCH net-next v2 2/3] net: use skb_for_each_frag() helper where possible
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    On 4/12/21 2:38 AM, Matteo Croce wrote:
    > From: Matteo Croce <mcroce@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++)


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