Re: [PATCH v2] ip_tunnel: disable preemption when updating per-cpu tstats

From: David Miller
Date: Mon Nov 16 2015 - 14:17:53 EST

From: "Jason A. Donenfeld" <Jason@xxxxxxxxx>
Date: Thu, 12 Nov 2015 17:35:58 +0100

> 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@xxxxxxxxx>

Applied and queued up for -stable, thanks Jason

Arguably, ip6tunnel_xmit() is primarily a ->ndo_start_xmit() and
therefore could assume that it only ran inside of a BH disabled code
sequence. And as you noted, when this was turned into a general case
helper function that guarantee was no longer necessarily there.

So another fix could have been to do local_bh_disable() in the
udp_tunnel6_xmit_skb() helper.

Thanks again.
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at