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Date: Fri, 15 Jan 2016 23:29:56 -0800 From: Eric Dumazet <eric.dumazet@...il.com> To: Craig Gallek <kraigatgoog@...il.com> Cc: Dmitry Vyukov <dvyukov@...gle.com>, "David S. Miller" <davem@...emloft.net>, netdev <netdev@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org> Subject: Re: net: hang in ip_finish_output On Fri, 2016-01-15 at 19:20 -0500, Craig Gallek wrote: > I wasn't able to reproduce this exact stack trace, but I was able to > cause soft lockup messages with a fork bomb of your test program. It > is certainly related to my recent SO_REUSEPORT change (reverting it > seems to fix the problem). I haven't completely figured out the exact > cause yet, though. Could you please post your configuration and > exactly how you are running this 'parallel loop'? There is a problem in the lookup functions (udp4_lib_lookup2() & __udp4_lib_lookup()) Because of RCU SLAB_DESTROY_BY_RCU semantics (check Documentation/RCU/rculist_nulls.txt for some details), you should not call reuseport_select_sock(sk, ...) without taking a stable reference on the sk socket. (and checking the lookup keys again) This is because sk could be freed, re-used by a totally different UDP socket on a different port, and the incoming frame(s) could be delivered on the wrong socket/channel/application :( Note that we discussed some time ago to remove SLAB_DESTROY_BY_RCU for UDP sockets (and freeing them after rcu grace period instead), so make UDP rx path faster, as we would no longer need to increment/decrement the socket refcount. This also would remove the added false sharing on sk_refcnt for the case the UDP socket serves as a tunnel (up->encap_rcv being non NULL)
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