linux-next test error: WARNING: suspicious RCU usage in ipmr_device_event

[syzbot <syzbot+21f82f61c24a7295edf5@syzkaller.appspotmail.com>]

Hello,

syzbot found the following crash on:

HEAD commit:    39a314cd Add linux-next specific files for 20200421
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=127ede73e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=8ef80c3f5d43f5bd
dashboard link: https://syzkaller.appspot.com/bug?extid=21f82f61c24a7295edf5
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+21f82f61c24a7295edf5@syzkaller.appspotmail.com

tipc: TX() has been purged, node left!
=============================
WARNING: suspicious RCU usage
5.7.0-rc2-next-20200421-syzkaller #0 Not tainted
-----------------------------
net/ipv4/ipmr.c:1757 RCU-list traversed in non-reader section!!

other info that might help us debug this:


rcu_scheduler_active = 2, debug_locks = 1
4 locks held by kworker/u4:0/7:
 #0: ffff8880a9772138 ((wq_completion)netns){+.+.}-{0:0}, at: __write_once_size include/linux/compiler.h:250 [inline]
 #0: ffff8880a9772138 ((wq_completion)netns){+.+.}-{0:0}, at: arch_atomic64_set arch/x86/include/asm/atomic64_64.h:34 [inline]
 #0: ffff8880a9772138 ((wq_completion)netns){+.+.}-{0:0}, at: atomic64_set include/asm-generic/atomic-instrumented.h:856 [inline]
 #0: ffff8880a9772138 ((wq_completion)netns){+.+.}-{0:0}, at: atomic_long_set include/asm-generic/atomic-long.h:41 [inline]
 #0: ffff8880a9772138 ((wq_completion)netns){+.+.}-{0:0}, at: set_work_data kernel/workqueue.c:615 [inline]
 #0: ffff8880a9772138 ((wq_completion)netns){+.+.}-{0:0}, at: set_work_pool_and_clear_pending kernel/workqueue.c:642 [inline]
 #0: ffff8880a9772138 ((wq_completion)netns){+.+.}-{0:0}, at: process_one_work+0x844/0x16a0 kernel/workqueue.c:2239
 #1: ffffc90000cdfdc0 (net_cleanup_work){+.+.}-{0:0}, at: process_one_work+0x878/0x16a0 kernel/workqueue.c:2243
 #2: ffffffff8a5a2b70 (pernet_ops_rwsem){++++}-{3:3}, at: cleanup_net+0x9b/0xa50 net/core/net_namespace.c:565
 #3: ffffffff8a5aeae8 (rtnl_mutex){+.+.}-{3:3}, at: ip6gre_exit_batch_net+0x88/0x700 net/ipv6/ip6_gre.c:1602

stack backtrace:
CPU: 0 PID: 7 Comm: kworker/u4:0 Not tainted 5.7.0-rc2-next-20200421-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: netns cleanup_net
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x188/0x20d lib/dump_stack.c:118
 ipmr_device_event+0x240/0x2b0 net/ipv4/ipmr.c:1757
 notifier_call_chain+0xc0/0x230 kernel/notifier.c:83
 call_netdevice_notifiers_info net/core/dev.c:1948 [inline]
 call_netdevice_notifiers_info+0xb5/0x130 net/core/dev.c:1933
 call_netdevice_notifiers_extack net/core/dev.c:1960 [inline]
 call_netdevice_notifiers net/core/dev.c:1974 [inline]
 rollback_registered_many+0x75c/0xe70 net/core/dev.c:8828
 unregister_netdevice_many.part.0+0x16/0x1e0 net/core/dev.c:9993
 unregister_netdevice_many+0x36/0x50 net/core/dev.c:9992
 ip6gre_exit_batch_net+0x4e8/0x700 net/ipv6/ip6_gre.c:1605
 ops_exit_list.isra.0+0x103/0x150 net/core/net_namespace.c:189
 cleanup_net+0x511/0xa50 net/core/net_namespace.c:603
 process_one_work+0x965/0x16a0 kernel/workqueue.c:2268
 worker_thread+0x96/0xe20 kernel/workqueue.c:2414
 kthread+0x38b/0x470 kernel/kthread.c:274
 ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352


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