Commit b4fe85f9 authored Nov 12, 2015 by Jason A. Donenfeld Committed by David S. Miller Nov 16, 2015

ip_tunnel: disable preemption when updating per-cpu tstats



Drivers like vxlan use the recently introduced
udp_tunnel_xmit_skb/udp_tunnel6_xmit_skb APIs. udp_tunnel6_xmit_skb
makes use of ip6tunnel_xmit, and ip6tunnel_xmit, after sending the
packet, updates the struct stats using the usual
u64_stats_update_begin/end calls on this_cpu_ptr(dev->tstats).
udp_tunnel_xmit_skb makes use of iptunnel_xmit, which doesn't touch
tstats, so drivers like vxlan, immediately after, call
iptunnel_xmit_stats, which does the same thing - calls
u64_stats_update_begin/end on this_cpu_ptr(dev->tstats).

While vxlan is probably fine (I don't know?), calling a similar function
from, say, an unbound workqueue, on a fully preemptable kernel causes
real issues:

[  188.434537] BUG: using smp_processor_id() in preemptible [00000000] code: kworker/u8:0/6
[  188.435579] caller is debug_smp_processor_id+0x17/0x20
[  188.435583] CPU: 0 PID: 6 Comm: kworker/u8:0 Not tainted 4.2.6 #2
[  188.435607] Call Trace:
[  188.435611]  [<ffffffff8234e936>] dump_stack+0x4f/0x7b
[  188.435615]  [<ffffffff81915f3d>] check_preemption_disabled+0x19d/0x1c0
[  188.435619]  [<ffffffff81915f77>] debug_smp_processor_id+0x17/0x20

The solution would be to protect the whole
this_cpu_ptr(dev->tstats)/u64_stats_update_begin/end blocks with
disabling preemption and then reenabling it.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 17c790a6

include/net/ip6_tunnel.h

+2 −1

Original line number	Diff line number	Diff line
		@@ -90,11 +90,12 @@ static inline void ip6tunnel_xmit(struct sock sk, struct sk_buff skb,
		err = ip6_local_out(dev_net(skb_dst(skb)->dev), sk, skb);

		if (net_xmit_eval(err) == 0) {
		struct pcpu_sw_netstats *tstats = this_cpu_ptr(dev->tstats);
		struct pcpu_sw_netstats *tstats = get_cpu_ptr(dev->tstats);
		u64_stats_update_begin(&tstats->syncp);
		tstats->tx_bytes += pkt_len;
		tstats->tx_packets++;
		u64_stats_update_end(&tstats->syncp);
		put_cpu_ptr(tstats);
		} else {
		stats->tx_errors++;
		stats->tx_aborted_errors++;

include/net/ip_tunnels.h

+2 −1

Original line number	Diff line number	Diff line
		@@ -287,12 +287,13 @@ static inline void iptunnel_xmit_stats(int err,
		struct pcpu_sw_netstats __percpu *stats)
		{
		if (err > 0) {
		struct pcpu_sw_netstats *tstats = this_cpu_ptr(stats);
		struct pcpu_sw_netstats *tstats = get_cpu_ptr(stats);

		u64_stats_update_begin(&tstats->syncp);
		tstats->tx_bytes += err;
		tstats->tx_packets++;
		u64_stats_update_end(&tstats->syncp);
		put_cpu_ptr(tstats);
		} else if (err < 0) {
		err_stats->tx_errors++;
		err_stats->tx_aborted_errors++;

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