Patch "ip_tunnel: disable preemption when updating per-cpu tstats" has been added to the 4.1-stable tree

This is a note to let you know that I've just added the patch titled

ip_tunnel: disable preemption when updating per-cpu tstats

to the 4.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
ip_tunnel-disable-preemption-when-updating-per-cpu-tstats.patch
and it can be found in the queue-4.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Jason A. Donenfeld" <[email protected]>
Date: Thu, 12 Nov 2015 17:35:58 +0100
Subject: ip_tunnel: disable preemption when updating per-cpu tstats

From: "Jason A. Donenfeld" <[email protected]>

[ Upstream commit b4fe85f9c9146f60457e9512fb6055e69e6a7a65 ]

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.

---
 include/net/ip6_tunnel.h |    3 ++-
 include/net/ip_tunnels.h |    3 ++-
 2 files changed, 4 insertions(+), 2 deletions(-)

@@ -83,11 +83,12 @@ static inline void ip6tunnel_xmit(struct
 	err = ip6_local_out_sk(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++;

@@ -207,12 +207,13 @@ static inline void iptunnel_xmit_stats(i
 				       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++;

Patches currently in stable-queue which might be from [email protected] are

queue-4.1/ip_tunnel-disable-preemption-when-updating-per-cpu-tstats.patch