ipvs: fix stats update from local clients

Local clients are not properly synchronized on 32-bit CPUs when
updating stats (3.10+). Now it is possible estimation_timer (timer),
a stats reader, to interrupt the local client in the middle of
write_seqcount_{begin,end} sequence leading to loop (DEADLOCK).
The same interrupt can happen from received packet (SoftIRQ)
which updates the same per-CPU stats.

Fix it by disabling BH while updating stats.

Found with debug:

WARNING: inconsistent lock state
4.17.0-rc2-00105-g35cb6d7-dirty #2 Not tainted
--------------------------------
inconsistent {IN-SOFTIRQ-R} -> {SOFTIRQ-ON-W} usage.
ftp/2545 [HC0[0]:SC0[0]:HE1:SE1] takes:
86845479 (&syncp->seq#6){+.+-}, at: ip_vs_schedule+0x1c5/0x59e [ip_vs]
{IN-SOFTIRQ-R} state was registered at:
 lock_acquire+0x44/0x5b
 estimation_timer+0x1b3/0x341 [ip_vs]
 call_timer_fn+0x54/0xcd
 run_timer_softirq+0x10c/0x12b
 __do_softirq+0xc1/0x1a9
 do_softirq_own_stack+0x1d/0x23
 irq_exit+0x4a/0x64
 smp_apic_timer_interrupt+0x63/0x71
 apic_timer_interrupt+0x3a/0x40
 default_idle+0xa/0xc
 arch_cpu_idle+0x9/0xb
 default_idle_call+0x21/0x23
 do_idle+0xa0/0x167
 cpu_startup_entry+0x19/0x1b
 start_secondary+0x133/0x182
 startup_32_smp+0x164/0x168
irq event stamp: 42213

other info that might help us debug this:
Possible unsafe locking scenario:

      CPU0
      ----
 lock(&syncp->seq#6);
 <Interrupt>
   lock(&syncp->seq#6);

*** DEADLOCK ***

Fixes: ac69269a45 ("ipvs: do not disable bh for long time")
Signed-off-by: Julian Anastasov <ja@ssi.bg>
Acked-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/ipvs/ip_vs_core.c

@@ -119,6 +119,8 @@ ip_vs_in_stats(struct ip_vs_conn *cp, struct sk_buff *skb)
 		struct ip_vs_cpu_stats *s;
 		struct ip_vs_service *svc;
 
+		local_bh_disable();
+
 		s = this_cpu_ptr(dest->stats.cpustats);
 		u64_stats_update_begin(&s->syncp);
 		s->cnt.inpkts++;
@@ -137,6 +139,8 @@ ip_vs_in_stats(struct ip_vs_conn *cp, struct sk_buff *skb)
 		s->cnt.inpkts++;
 		s->cnt.inbytes += skb->len;
 		u64_stats_update_end(&s->syncp);
+
+		local_bh_enable();
 	}
 }
 
@@ -151,6 +155,8 @@ ip_vs_out_stats(struct ip_vs_conn *cp, struct sk_buff *skb)
 		struct ip_vs_cpu_stats *s;
 		struct ip_vs_service *svc;
 
+		local_bh_disable();
+
 		s = this_cpu_ptr(dest->stats.cpustats);
 		u64_stats_update_begin(&s->syncp);
 		s->cnt.outpkts++;
@@ -169,6 +175,8 @@ ip_vs_out_stats(struct ip_vs_conn *cp, struct sk_buff *skb)
 		s->cnt.outpkts++;
 		s->cnt.outbytes += skb->len;
 		u64_stats_update_end(&s->syncp);
+
+		local_bh_enable();
 	}
 }
 
@@ -179,6 +187,8 @@ ip_vs_conn_stats(struct ip_vs_conn *cp, struct ip_vs_service *svc)
 	struct netns_ipvs *ipvs = svc->ipvs;
 	struct ip_vs_cpu_stats *s;
 
+	local_bh_disable();
+
 	s = this_cpu_ptr(cp->dest->stats.cpustats);
 	u64_stats_update_begin(&s->syncp);
 	s->cnt.conns++;
@@ -193,6 +203,8 @@ ip_vs_conn_stats(struct ip_vs_conn *cp, struct ip_vs_service *svc)
 	u64_stats_update_begin(&s->syncp);
 	s->cnt.conns++;
 	u64_stats_update_end(&s->syncp);
+
+	local_bh_enable();
 }