tcp: use ACCESS_ONCE() in tcp_update_pacing_rate()

sk_pacing_rate is read by sch_fq packet scheduler at any time,
with no synchronization, so make sure we update it in a
sensible way. ACCESS_ONCE() is how we instruct compiler
to not do stupid things, like using the memory location
as a temporary variable.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 1 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 47b8ab7dce9c..eb651a069a6c 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -755,7 +755,12 @@ static void tcp_update_pacing_rate(struct sock *sk)
         if (tp->srtt > 8 + 2)
                 do_div(rate, tp->srtt);
 
-        sk->sk_pacing_rate = min_t(u64, rate, sk->sk_max_pacing_rate);
+        /* ACCESS_ONCE() is needed because sch_fq fetches sk_pacing_rate
+         * without any lock. We want to make sure compiler wont store
+         * intermediate values in this location.
+         */
+        ACCESS_ONCE(sk->sk_pacing_rate) = min_t(u64, rate,
+                                                sk->sk_max_pacing_rate);
 }
 
 /* Calculate rto without backoff.  This is the second half of Van Jacobson's