对udp dns的一次思考

int udp_v4_get_port(struct sock *sk, unsigned short snum)
{
    unsigned int hash2_nulladdr =
        udp4_portaddr_hash(sock_net(sk), htonl(INADDR_ANY), snum);
    unsigned int hash2_partial =
        udp4_portaddr_hash(sock_net(sk), inet_sk(sk)->inet_rcv_saddr, 0);

    /* precompute partial secondary hash */
    udp_sk(sk)->udp_portaddr_hash = hash2_partial;
    return udp_lib_get_port(sk, snum, ipv4_rcv_saddr_equal, hash2_nulladdr);
}

  1 int udp_lib_get_port(struct sock *sk, unsigned short snum,
  2              int (*saddr_comp)(const struct sock *sk1,
  3                        const struct sock *sk2,
  4                        bool match_wildcard),
  5              unsigned int hash2_nulladdr)
  6 {
  7     struct udp_hslot *hslot, *hslot2;
  8     struct udp_table *udptable = sk->sk_prot->h.udp_table;
  9     int    error = 1;
 10     struct net *net = sock_net(sk);
 11 
 12     if (!snum) {
 13         int low, high, remaining;
 14         unsigned int rand;
 15         unsigned short first, last;
 16         DECLARE_BITMAP(bitmap, PORTS_PER_CHAIN);
 17 
 18         inet_get_local_port_range(net, &low, &high);
 19         remaining = (high - low) + 1;
 20 
 21         rand = prandom_u32();
 22         first = reciprocal_scale(rand, remaining) + low;
 23         /*
 24          * force rand to be an odd multiple of UDP_HTABLE_SIZE
 25          */
 26         rand = (rand | 1) * (udptable->mask + 1);
 27         last = first + udptable->mask + 1;
 28         do {
 29             hslot = udp_hashslot(udptable, net, first);
 30             bitmap_zero(bitmap, PORTS_PER_CHAIN);
 31             spin_lock_bh(&hslot->lock);
 32             udp_lib_lport_inuse(net, snum, hslot, bitmap, sk,
 33                         saddr_comp, udptable->log);
 34 
 35             snum = first;
 36             /*
 37              * Iterate on all possible values of snum for this hash.
 38              * Using steps of an odd multiple of UDP_HTABLE_SIZE
 39              * give us randomization and full range coverage.
 40              */
 41             do {
 42                 if (low <= snum && snum <= high &&
 43                     !test_bit(snum >> udptable->log, bitmap) &&
 44                     !inet_is_local_reserved_port(net, snum))
 45                     goto found;
 46                 snum += rand;
 47             } while (snum != first);
 48             spin_unlock_bh(&hslot->lock);
 49         } while (++first != last);
 50         goto fail;
 51     } else {
 52         hslot = udp_hashslot(udptable, net, snum);
 53         spin_lock_bh(&hslot->lock);
 54         if (hslot->count > 10) {
 55             int exist;
 56             unsigned int slot2 = udp_sk(sk)->udp_portaddr_hash ^ snum;
 57 
 58             slot2          &= udptable->mask;
 59             hash2_nulladdr &= udptable->mask;
 60 
 61             hslot2 = udp_hashslot2(udptable, slot2);
 62             if (hslot->count < hslot2->count)
 63                 goto scan_primary_hash;
 64 
 65             exist = udp_lib_lport_inuse2(net, snum, hslot2,
 66                              sk, saddr_comp);
 67             if (!exist && (hash2_nulladdr != slot2)) {
 68                 hslot2 = udp_hashslot2(udptable, hash2_nulladdr);
 69                 exist = udp_lib_lport_inuse2(net, snum, hslot2,
 70                                  sk, saddr_comp);
 71             }
 72             if (exist)
 73                 goto fail_unlock;
 74             else
 75                 goto found;
 76         }
 77 scan_primary_hash:
 78         if (udp_lib_lport_inuse(net, snum, hslot, NULL, sk,
 79                     saddr_comp, 0))
 80             goto fail_unlock;
 81     }
 82 found:
 83     inet_sk(sk)->inet_num = snum;
 84     udp_sk(sk)->udp_port_hash = snum;
 85     udp_sk(sk)->udp_portaddr_hash ^= snum;
 86     if (sk_unhashed(sk)) {
 87         if (sk->sk_reuseport &&
 88             udp_reuseport_add_sock(sk, hslot, saddr_comp)) {
 89             inet_sk(sk)->inet_num = 0;
 90             udp_sk(sk)->udp_port_hash = 0;
 91             udp_sk(sk)->udp_portaddr_hash ^= snum;
 92             goto fail_unlock;
 93         }
 94 
 95         sk_add_node_rcu(sk, &hslot->head);
 96         hslot->count++;
 97         sock_prot_inuse_add(sock_net(sk), sk->sk_prot, 1);
 98 
 99         hslot2 = udp_hashslot2(udptable, udp_sk(sk)->udp_portaddr_hash);
100         spin_lock(&hslot2->lock);
101         if (IS_ENABLED(CONFIG_IPV6) && sk->sk_reuseport &&
102             sk->sk_family == AF_INET6)
103             hlist_add_tail_rcu(&udp_sk(sk)->udp_portaddr_node,
104                        &hslot2->head);
105         else
106             hlist_add_head_rcu(&udp_sk(sk)->udp_portaddr_node,
107                        &hslot2->head);
108         hslot2->count++;
109         spin_unlock(&hslot2->lock);
110     }
111     sock_set_flag(sk, SOCK_RCU_FREE);
112     error = 0;
113 fail_unlock:
114     spin_unlock_bh(&hslot->lock);
115 fail:
116     return error;
117 }