net, phonet, rcu: rcu hang within gprs_attach

Hi all,

While fuzzing with trinity inside a KVM tools guest running the latest -next
kernel I've stumbled on the following stack trace (full log attached):

[  370.662014] INFO: task trinity-main:8727 blocked for more than 120 seconds.
[  370.662891]       Not tainted 3.16.0-rc6-next-20140724-sasha-00046-g7324c87-dirty #932
[  370.663655] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[  370.664562] trinity-main    D ffff88053cc80000 13064  8727   8714 0x00000000
[  370.665328]  ffff88053da6fc10 0000000000000002 ffff8805483e2dc8 ffff880541873000
[  370.666147]  000000276ed30787 ffff88053da6c010 ffff88053da6c000 ffff8805452a0000
[  370.667243]  ffff880541873000 0000000000000000 7fffffffffffffff ffffffffb3ec51d8
[  370.668788] Call Trace:
[  370.669118] schedule (kernel/sched/core.c:2847)
[  370.670538] schedule_timeout (kernel/time/timer.c:1476)
[  370.671524] ? mark_lock (kernel/locking/lockdep.c:2894)
[  370.672299] ? __this_cpu_preempt_check (lib/smp_processor_id.c:63)
[  370.673227] ? get_parent_ip (kernel/sched/core.c:2561)
[  370.674085] wait_for_completion (include/linux/spinlock.h:328 kernel/sched/completion.c:76 kernel/sched/completion.c:93 kernel/sched/completion.c:101 kernel/sched/completion.c:122)
[  370.674960] ? wake_up_state (kernel/sched/core.c:2942)
[  370.675576] _rcu_barrier (kernel/rcu/tree.c:3325 (discriminator 8))
[  370.676109] rcu_barrier (kernel/rcu/tree_plugin.h:920)
[  370.676627] netdev_run_todo (net/core/dev.c:6323)
[  370.677202] rtnl_unlock (net/core/rtnetlink.c:80)
[  370.677714] unregister_netdev (net/core/dev.c:6687)
[  370.678266] gprs_attach (net/phonet/pep-gprs.c:311)
[  370.679641] pep_setsockopt (net/phonet/pep.c:1016)
[  370.681082] sock_common_setsockopt (net/core/sock.c:2603)
[  370.682048] SyS_setsockopt (net/socket.c:1914 net/socket.c:1894)
[  370.682854] tracesys (arch/x86/kernel/entry_64.S:541)
[  370.683586] 1 lock held by trinity-main/8727:
[  370.684232] #0: (rcu_preempt_state.barrier_mutex){+.+...}, at: _rcu_barrier (kernel/rcu/tree.c:3233)

This has reproduced couple of times, and has always originated from gprs_attach. I don't see any obvious
issues with the code there, so I'm not sure if it's a fault of the phonet or the rcu code.


Thanks,
Sasha