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>

1 files changed, 14 insertions, 4 deletions

diff --git a/include/net/tcp.h b/include/net/tcp.h
index df6a2eb2019..eaa9582779d 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 */