tcp: Combat per-cpu skew in orphan tests.

As reported by Anton Blanchard when we use
percpu_counter_read_positive() to make our orphan socket limit checks,
the check can be off by up to num_cpus_online() * batch (which is 32
by default) which on a 128 cpu machine can be as large as the default
orphan limit itself.

Fix this by doing the full expensive sum check if the optimized check
triggers.

Reported-by: Anton Blanchard <anton@samba.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>

3 files changed, 19 insertions, 12 deletions

diff --git a/include/net/tcp.h b/include/net/tcp.h
index df6a2eb20193..eaa9582779d0 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -268,11 +268,21 @@ static inline int between(__u32 seq1, __u32 seq2, __u32 seq3)
         return seq3 - seq2 >= seq1 - seq2;
 }
 
-static inline int tcp_too_many_orphans(struct sock *sk, int num)
+static inline bool tcp_too_many_orphans(struct sock *sk, int shift)
 {
-        return (num > sysctl_tcp_max_orphans) ||
-                (sk->sk_wmem_queued > SOCK_MIN_SNDBUF &&
-                 atomic_read(&tcp_memory_allocated) > sysctl_tcp_mem[2]);
+        struct percpu_counter *ocp = sk->sk_prot->orphan_count;
+        int orphans = percpu_counter_read_positive(ocp);
+
+        if (orphans << shift > sysctl_tcp_max_orphans) {
+                orphans = percpu_counter_sum_positive(ocp);
+                if (orphans << shift > sysctl_tcp_max_orphans)
+                        return true;
+        }
+
+        if (sk->sk_wmem_queued > SOCK_MIN_SNDBUF &&
+            atomic_read(&tcp_memory_allocated) > sysctl_tcp_mem[2])
+                return true;
+        return false;
 }
 
 /* syncookies: remember time of last synqueue overflow */
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 176e11aaea77..197b9b77fa3e 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2011,11 +2011,8 @@ adjudge_to_death:
                 }
         }
         if (sk->sk_state != TCP_CLOSE) {
-                int orphan_count = percpu_counter_read_positive(
-                                                sk->sk_prot->orphan_count);
-
                 sk_mem_reclaim(sk);
-                if (tcp_too_many_orphans(sk, orphan_count)) {
+                if (tcp_too_many_orphans(sk, 0)) {
                         if (net_ratelimit())
                                 printk(KERN_INFO "TCP: too many of orphaned "
                                        "sockets\n");
diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 808bb920c9f5..c35b469e851c 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -66,18 +66,18 @@ static void tcp_write_err(struct sock *sk)
 static int tcp_out_of_resources(struct sock *sk, int do_reset)
 {
         struct tcp_sock *tp = tcp_sk(sk);
-        int orphans = percpu_counter_read_positive(&tcp_orphan_count);
+        int shift = 0;
 
         /* If peer does not open window for long time, or did not transmit
          * anything for long time, penalize it. */
         if ((s32)(tcp_time_stamp - tp->lsndtime) > 2*TCP_RTO_MAX || !do_reset)
-                orphans <<= 1;
+                shift++;
 
         /* If some dubious ICMP arrived, penalize even more. */
         if (sk->sk_err_soft)
-                orphans <<= 1;
+                shift++;
 
-        if (tcp_too_many_orphans(sk, orphans)) {
+        if (tcp_too_many_orphans(sk, shift)) {
                 if (net_ratelimit())
                         printk(KERN_INFO "Out of socket memory\n");