[EVL] Kernel WARNING: notifier callback netevent_handler already registered

[Hannes Diethelm <hannes.diethelm@gmail.com>]

Hello

There is a Kernel WARNING: "notifier callback netevent_handler already registered right after boot". It is repeated 5 times.

Config:
Debian Trixie with xfce4 Desktop
libevl: r56
linux-evl: v6.12.67-evl2-rebase

I traced the issue already trough the following code:
net/core/net_namespace.c:355
kernel/evl/net/net.c:35
kernel/evl/net/ipv4/ipv4.c:41
kernel/evl/net/ipv4/arp.c:323

However, I lack the knowledge to do a proper fix. One way would be to just check in evl_net_init_arp() if it is already
registered and don't do it more than once but that doesn't feel right.

Some background: I am working on integrating Xenomai3/4 into LinuxCNC. Due to this, I have a full desktop running with a
Xenomai kernel. Probably one of the installed applications triggers this issue.

It is just a hobby and I am not an official maintainer of LinuxCNC but if it works well, it might get merged.

Regards
Hannes

Full warning, one of 5 but they are all equal:

[    4.158674] ------------[ cut here ]------------
[    4.158680] notifier callback netevent_handler already registered
[    4.158716] WARNING: CPU: 15 PID: 1095 at kernel/notifier.c:31 notifier_chain_register+0x3e/0xb0
[    4.158736] Modules linked in: binfmt_misc intel_rapl_msr intel_rapl_common ccp kvm irqbypass crct10dif_pclmul ghash_clmulni_intel snd_hda_codec_generic sha512_ssse3 sha256_ssse3 sha1_ssse3 snd_hda_intel aesni_intel snd_intel_dspcfg snd_intel_sdw_acpi gf128mul snd_hda_codec crypto_simd cryptd pcspkr snd_hda_core snd_hwdep snd_pcm snd_timer snd soundcore evdev sg parport_pc ppdev lp parport efi_pstore configfs nfnetlink vsock_loopback vmw_vsock_virtio_transport_common vmw_vsock_vmci_transport vsock vmw_vmci qemu_fw_cfg virtio_rng ip_tables x_tables autofs4 ext4 crc32c_generic crc16 mbcache jbd2 virtio_gpu virtio_dma_buf drm_shmem_helper sr_mod drm_kms_helper cdrom iTCO_wdt intel_pmc_bxt xhci_pci iTCO_vendor_support watchdog xhci_hcd ahci libahci drm crc32_pclmul usbcore crc32c_intel virtio_blk libata psmouse virtio_net net_failover serio_raw failover usb_common scsi_mod i2c_i801 lpc_ich i2c_smbus scsi_common button
[    4.158773] CPU: 15 UID: 0 PID: 1095 Comm: (crub_all) Not tainted 6.12.67-xenomai4-53a3aa #3
[    4.158777] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[    4.158778] IRQ stage: Linux
[    4.158781] RIP: 0010:notifier_chain_register+0x3e/0xb0
[    4.158784] Code: 10 7f 33 75 04 84 d2 75 7d 48 8d 78 08 48 8b 40 08 48 85 c0 74 20 48 39 c6 75 e0 48 8b 36 48 c7 c7 98 e1 54 9f e8 b2 28 fc ff <0f> 0b b8 ef ff ff ff c3 cc cc cc cc 48 89 46 08 48 89 37 66 90 31
[    4.158785] RSP: 0018:ffffc91b414e7d00 EFLAGS: 00010282
[    4.158789] RAX: 0000000000000000 RBX: ffffffffa090b240 RCX: 0000000000000027
[    4.158793] RDX: ffff8901efda0948 RSI: 0000000000000001 RDI: ffff8901efda0940
[    4.158794] RBP: ffffffff9fe809c0 R08: 0000000000000a74 R09: 0000000000000000
[    4.158794] R10: 0000000000000001 R11: 0000000000000000 R12: ffffffff9fc5a3e0
[    4.158795] R13: 0000000000000000 R14: ffffffff9fc528ec R15: ffffc91b414e7d58
[    4.158797] FS:  00007f16c63765c0(0000) GS:ffff8901efd80000(0000) knlGS:0000000000000000
[    4.158801] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    4.158802] CR2: 00007f16c719f160 CR3: 000000010491c000 CR4: 0000000000750ef0
[    4.158803] PKRU: 55555554
[    4.158803] Call Trace:
[    4.158815]  <TASK>
[    4.158816]  atomic_notifier_chain_register+0x24/0x40
[    4.158820]  evl_net_init_arp+0x3f/0x50
[    4.158830]  evl_net_init_ipv4+0x2a/0x100
[    4.158833]  setup_net+0x9e/0x2b0
[    4.158858]  copy_net_ns+0x111/0x270
[    4.158861]  create_new_namespaces+0x113/0x2e0
[    4.158865]  unshare_nsproxy_namespaces+0x58/0xa0
[    4.158868]  ksys_unshare+0x17d/0x400
[    4.158887]  __x64_sys_unshare+0x12/0x20
[    4.158890]  do_syscall_64+0xc0/0x230
[    4.158913]  ? syscall_exit_to_user_mode+0x1da/0x1f0
[    4.158916]  ? do_syscall_64+0xcc/0x230
[    4.158917]  ? do_user_addr_fault+0x4c1/0x750
[    4.158926]  ? __ct_user_enter+0x29/0xd0
[    4.158927]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
[    4.158949] RIP: 0033:0x7f16c6ebe967
[    4.158960] Code: 73 01 c3 48 8b 0d a9 34 0d 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 b8 10 01 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 79 34 0d 00 f7 d8 64 89 01 48
[    4.158961] RSP: 002b:00007ffe188cc308 EFLAGS: 00000246 ORIG_RAX: 0000000000000110
[    4.158962] RAX: ffffffffffffffda RBX: 00007ffe188cc8c8 RCX: 00007f16c6ebe967
[    4.158962] RDX: 0000000000000000 RSI: 00007ffe188cc270 RDI: 0000000040000000
[    4.158963] RBP: 00007ffe188cc350 R08: 0000000000000000 R09: 0000000000000000
[    4.158963] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000040000000
[    4.158967] R13: 00000000fffffff5 R14: 00007f16c7374380 R15: 00000000fffffff5
[    4.158969]  </TASK>
[    4.158969] ---[ end trace 0000000000000000 ]---