net/sched: act_police: fix race condition on state variables

after 'police' configuration parameters were converted to use RCU instead
of spinlock, the state variables used to compute the traffic rate (namely
'tcfp_toks', 'tcfp_ptoks' and 'tcfp_t_c') are erroneously read/updated in
the traffic path without any protection.

Use a dedicated spinlock to avoid race conditions on these variables, and
ensure proper cache-line alignment. In this way, 'police' is still faster
than what we observed when 'tcf_lock' was used in the traffic path _ i.e.
reverting commit 2d550dba ("net/sched: act_police: don't use spinlock
in the data path"). Moreover, we preserve the throughput improvement that
was obtained after 'police' started using per-cpu counters, when 'avrate'
is used instead of 'rate'.

Changes since v1 (thanks to Eric Dumazet):
- call ktime_get_ns() before acquiring the lock in the traffic path
- use a dedicated spinlock instead of tcf_lock
- improve cache-line usage

Fixes: 2d550dba ("net/sched: act_police: don't use spinlock in the data path")
Reported-and-suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>

net/sched/act_police.c

@@ -27,10 +27,7 @@ struct tcf_police_params {
 	u32			tcfp_ewma_rate;
 	s64			tcfp_burst;
 	u32			tcfp_mtu;
-	s64			tcfp_toks;
-	s64			tcfp_ptoks;
 	s64			tcfp_mtu_ptoks;
-	s64			tcfp_t_c;
 	struct psched_ratecfg	rate;
 	bool			rate_present;
 	struct psched_ratecfg	peak;
@@ -41,6 +38,11 @@ struct tcf_police_params {
 struct tcf_police {
 	struct tc_action	common;
 	struct tcf_police_params __rcu *params;
+
+	spinlock_t		tcfp_lock ____cacheline_aligned_in_smp;
+	s64			tcfp_toks;
+	s64			tcfp_ptoks;
+	s64			tcfp_t_c;
 };
 
 #define to_police(pc) ((struct tcf_police *)pc)
@@ -186,12 +188,9 @@ static int tcf_police_init(struct net *net, struct nlattr *nla,
 	}
 
 	new->tcfp_burst = PSCHED_TICKS2NS(parm->burst);
-	new->tcfp_toks = new->tcfp_burst;
-	if (new->peak_present) {
+	if (new->peak_present)
 		new->tcfp_mtu_ptoks = (s64)psched_l2t_ns(&new->peak,
 							 new->tcfp_mtu);
-		new->tcfp_ptoks = new->tcfp_mtu_ptoks;
-	}
 
 	if (tb[TCA_POLICE_AVRATE])
 		new->tcfp_ewma_rate = nla_get_u32(tb[TCA_POLICE_AVRATE]);
@@ -207,7 +206,12 @@ static int tcf_police_init(struct net *net, struct nlattr *nla,
 	}
 
 	spin_lock_bh(&police->tcf_lock);
-	new->tcfp_t_c = ktime_get_ns();
+	spin_lock_bh(&police->tcfp_lock);
+	police->tcfp_t_c = ktime_get_ns();
+	police->tcfp_toks = new->tcfp_burst;
+	if (new->peak_present)
+		police->tcfp_ptoks = new->tcfp_mtu_ptoks;
+	spin_unlock_bh(&police->tcfp_lock);
 	police->tcf_action = parm->action;
 	rcu_swap_protected(police->params,
 			   new,
@@ -257,25 +261,28 @@ static int tcf_police_act(struct sk_buff *skb, const struct tc_action *a,
 		}
 
 		now = ktime_get_ns();
-		toks = min_t(s64, now - p->tcfp_t_c, p->tcfp_burst);
+		spin_lock_bh(&police->tcfp_lock);
+		toks = min_t(s64, now - police->tcfp_t_c, p->tcfp_burst);
 		if (p->peak_present) {
-			ptoks = toks + p->tcfp_ptoks;
+			ptoks = toks + police->tcfp_ptoks;
 			if (ptoks > p->tcfp_mtu_ptoks)
 				ptoks = p->tcfp_mtu_ptoks;
 			ptoks -= (s64)psched_l2t_ns(&p->peak,
 						    qdisc_pkt_len(skb));
 		}
-		toks += p->tcfp_toks;
+		toks += police->tcfp_toks;
 		if (toks > p->tcfp_burst)
 			toks = p->tcfp_burst;
 		toks -= (s64)psched_l2t_ns(&p->rate, qdisc_pkt_len(skb));
 		if ((toks|ptoks) >= 0) {
-			p->tcfp_t_c = now;
-			p->tcfp_toks = toks;
-			p->tcfp_ptoks = ptoks;
+			police->tcfp_t_c = now;
+			police->tcfp_toks = toks;
+			police->tcfp_ptoks = ptoks;
+			spin_unlock_bh(&police->tcfp_lock);
 			ret = p->tcfp_result;
 			goto inc_drops;
 		}
+		spin_unlock_bh(&police->tcfp_lock);
 	}
 
 inc_overlimits: