Re: "Dead loop on virtual device" error without softirq-BKL on PREEMPT_RT
From: Bert Karwatzki
Date: Tue Feb 17 2026 - 03:57:04 EST
Am Dienstag, dem 17.02.2026 um 08:19 +0100 schrieb Sebastian Andrzej Siewior:
> On 2026-02-17 00:48:25 [+0100], Bert Karwatzki wrote:
> > The problem seems to be that different preemtible threads try to send skbs.
>
> This does not matter because the counter is per-thread not per-CPU.
The "Dead loop on virtual device" messages is not printed because dev_xmit_recursion()
returns true, but because READ_ONCE(txq->xmit_lock_owner) == cpu.
>
> > 2026-02-17T00:29:24.212231+01:00 [ T1513] Dead loop on virtual device wlp4s0, fix it urgently!
>
> Could you please do a backtrace here, for instance via WARN_ON_ONCE()
>
> > Bert Karwatzki
>
> Sebastian
Here's the backtrace:
2026-02-17T09:48:49.553225+01:00 lisa kernel: [ T1521] __dev_queue_xmit 0: skb = ffff9ee68afd3a00 dev = wlp4s0
2026-02-17T09:48:49.553227+01:00 lisa kernel: [ T1521] __dev_queue_xmit 2: skb = ffff9ee68afd3a00 dev = wlp4s0 txq = ffff9ee6820b5a00
2026-02-17T09:48:49.553229+01:00 lisa kernel: [ T1521] __dev_queue_xmit: cpu = 7 xmit_lock_owner = -1
2026-02-17T09:48:49.553232+01:00 lisa kernel: [ T1521] __dev_queue_xmit 3: skb = ffff9ee68afd3a00 dev = wlp4s0 txq = ffff9ee6820b5a00
2026-02-17T09:48:49.553234+01:00 lisa kernel: [ T1521] __dev_queue_xmit 4: skb = ffff9ee68afd3a00 dev = wlp4s0 txq = ffff9ee6820b5a00
2026-02-17T09:48:49.553235+01:00 lisa kernel: [ T1538] __dev_queue_xmit 0: skb = ffff9ee68afd3b00 dev = wlp4s0
2026-02-17T09:48:49.553238+01:00 lisa kernel: [ T1538] __dev_queue_xmit 2: skb = ffff9ee68afd3b00 dev = wlp4s0 txq = ffff9ee6820b5a00
2026-02-17T09:48:49.553241+01:00 lisa kernel: [ T1538] __dev_queue_xmit: cpu = 7 xmit_lock_owner = 7
2026-02-17T09:48:49.553243+01:00 lisa kernel: [ T1538] ------------[ cut here ]------------
2026-02-17T09:48:49.553245+01:00 lisa kernel: [ T1538] WARNING: CPU: 7 PID: 1538 at net/core/dev.c:4800 __dev_queue_xmit.cold+0x163/0x5bd
2026-02-17T09:48:49.553248+01:00 lisa kernel: [ T1538] Modules linked in: ccm snd_seq_dummy snd_hrtimer snd_seq_midi snd_seq_midi_event snd_rawmidi snd_seq
snd_seq_device rfcomm bnep nls_ascii nls_cp437 vfat fat snd_hda_codec_generic snd_hda_codec_atihdmi snd_hda_codec_hdmi snd_hda_intel btusb snd_intel_dspcfg
btrtl uvcvideo btintel snd_hda_codec btbcm videobuf2_vmalloc snd_acp3x_pdm_dma snd_soc_dmic snd_acp3x_rn btmtk videobuf2_memops snd_soc_core snd_hda_core uvc
videobuf2_v4l2 bluetooth snd_hwdep videodev snd_pcm_oss snd_mixer_oss snd_pcm snd_rn_pci_acp3x snd_acp_config videobuf2_common msi_wmi snd_soc_acpi ecdh_generic
ecc mc sparse_keymap wmi_bmof snd_timer snd k10temp ccp soundcore snd_pci_acp3x battery ac button joydev evdev amd_pmc mt7921e mt7921_common mt792x_lib
mt76_connac_lib mt76 mac80211 libarc4 cfg80211 rfkill msr fuse nvme_fabrics efi_pstore configfs efivarfs autofs4 ext4 mbcache jbd2 usbhid amdgpu drm_client_lib
i2c_algo_bit drm_ttm_helper ttm drm_exec drm_suballoc_helper drm_buddy drm_panel_backlight_quirks gpu_sched xhci_pci amdxcp xhci_hcd
2026-02-17T09:48:49.553251+01:00 lisa kernel: [ T1538] drm_display_helper hid_sensor_hub hid_multitouch mfd_core hid_generic i2c_hid_acpi psmouse
drm_kms_helper usbcore nvme amd_sfh i2c_hid hid cec nvme_core serio_raw i2c_piix4 r8169 crc16 usb_common i2c_smbus i2c_designware_platform i2c_designware_core
2026-02-17T09:48:49.553254+01:00 lisa kernel: [ T1538] CPU: 7 UID: 122 PID: 1538 Comm: isc-loop-0014 Not tainted 6.18.10-deadloop-00005-g56089d3b695a #1153
PREEMPT_{RT,(full)}
2026-02-17T09:48:49.553256+01:00 lisa kernel: [ T1538] Hardware name: Micro-Star International Co., Ltd. Alpha 15 B5EEK/MS-158L, BIOS E158LAMS.10F 11/11/2024
2026-02-17T09:48:49.553258+01:00 lisa kernel: [ T1538] RIP: 0010:__dev_queue_xmit.cold+0x163/0x5bd
2026-02-17T09:48:49.553261+01:00 lisa kernel: [ T1538] Code: 3d 3e 88 5a 01 66 41 83 bf ca 04 00 00 08 0f 86 4d 02 00 00 4c 89 f2 48 c7 c6 80 b3 0c b9 48 c7 c7
b0 b3 2f b9 e8 49 43 fd ff <0f> 0b e8 72 8f 75 00 85 c0 74 0f 4c 89 f6 48 c7 c7 58 b4 2f b9 e8
2026-02-17T09:48:49.553262+01:00 lisa kernel: [ T1538] RSP: 0018:ffffbd6506d53990 EFLAGS: 00010246
2026-02-17T09:48:49.553264+01:00 lisa kernel: [ T1538] RAX: 0000000000000007 RBX: ffff9ee6a089c000 RCX: 0000000000000027
2026-02-17T09:48:49.553265+01:00 lisa kernel: [ T1538] RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffff9ee95e7d6d80
2026-02-17T09:48:49.553266+01:00 lisa kernel: [ T1538] RBP: ffff9ee68afd3b00 R08: 0000000000000000 R09: ffffffffb9685590
2026-02-17T09:48:49.553268+01:00 lisa kernel: [ T1538] R10: ffffffffb96a4210 R11: 0000000000000003 R12: ffff9ee6820b5a00
2026-02-17T09:48:49.553270+01:00 lisa kernel: [ T1538] R13: 0000000000000007 R14: ffff9ee6a089c118 R15: ffff9ee6b455f000
2026-02-17T09:48:49.553273+01:00 lisa kernel: [ T1538] FS: 00007f3ad75ff680(0000) GS:ffff9ee9a4bf3000(0000) knlGS:0000000000000000
2026-02-17T09:48:49.553275+01:00 lisa kernel: [ T1538] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
2026-02-17T09:48:49.553277+01:00 lisa kernel: [ T1538] CR2: 00007f3d54a24080 CR3: 0000000102f73000 CR4: 0000000000750ef0
2026-02-17T09:48:49.553279+01:00 lisa kernel: [ T1538] PKRU: 55555554
2026-02-17T09:48:49.553282+01:00 lisa kernel: [ T1538] Call Trace:
2026-02-17T09:48:49.553284+01:00 lisa kernel: [ T1538] <TASK>
2026-02-17T09:48:49.553285+01:00 lisa kernel: [ T1538] ? srso_alias_return_thunk+0x5/0xfbef5
2026-02-17T09:48:49.553287+01:00 lisa kernel: [ T1538] ? __ip_make_skb+0x325/0x560
2026-02-17T09:48:49.553289+01:00 lisa kernel: [ T1538] ip_finish_output2+0x2c8/0x600
2026-02-17T09:48:49.553292+01:00 lisa kernel: [ T1538] ip_local_out+0xc6/0xf0
2026-02-17T09:48:49.553294+01:00 lisa kernel: [ T1538] ip_send_skb+0x14/0x50
2026-02-17T09:48:49.553296+01:00 lisa kernel: [ T1538] udp_send_skb+0x181/0x370
2026-02-17T09:48:49.553299+01:00 lisa kernel: [ T1538] udp_sendmsg+0x8e8/0xbc0
2026-02-17T09:48:49.553301+01:00 lisa kernel: [ T1538] ? ip_frag_init+0x60/0x60
2026-02-17T09:48:49.553302+01:00 lisa kernel: [ T1538] ? srso_alias_return_thunk+0x5/0xfbef5
2026-02-17T09:48:49.553304+01:00 lisa kernel: [ T1538] ? aa_sk_perm+0x8e/0x210
2026-02-17T09:48:49.553306+01:00 lisa kernel: [ T1538] __sock_sendmsg+0x60/0x80
2026-02-17T09:48:49.553308+01:00 lisa kernel: [ T1538] ____sys_sendmsg+0x21d/0x2b0
2026-02-17T09:48:49.553310+01:00 lisa kernel: [ T1538] ? import_iovec+0x1b/0x30
2026-02-17T09:48:49.553312+01:00 lisa kernel: [ T1538] ? srso_alias_return_thunk+0x5/0xfbef5
2026-02-17T09:48:49.553315+01:00 lisa kernel: [ T1538] ? copy_msghdr_from_user+0xe5/0x170
2026-02-17T09:48:49.553316+01:00 lisa kernel: [ T1538] ___sys_sendmsg+0x7e/0xc0
2026-02-17T09:48:49.553318+01:00 lisa kernel: [ T1538] ? srso_alias_return_thunk+0x5/0xfbef5
2026-02-17T09:48:49.553318+01:00 lisa kernel: [ T1538] ? rt_spin_lock+0x38/0x110
2026-02-17T09:48:49.553321+01:00 lisa kernel: [ T1538] ? srso_alias_return_thunk+0x5/0xfbef5
2026-02-17T09:48:49.553323+01:00 lisa kernel: [ T1538] ? rt_spin_lock+0x38/0x110
2026-02-17T09:48:49.553325+01:00 lisa kernel: [ T1538] ? srso_alias_return_thunk+0x5/0xfbef5
2026-02-17T09:48:49.553327+01:00 lisa kernel: [ T1538] ? ipv4_dst_check+0x36/0x60
2026-02-17T09:48:49.553330+01:00 lisa kernel: [ T1538] ? srso_alias_return_thunk+0x5/0xfbef5
2026-02-17T09:48:49.553332+01:00 lisa kernel: [ T1538] ? ip4_datagram_release_cb+0x45/0x1d0
2026-02-17T09:48:49.553334+01:00 lisa kernel: [ T1538] ? rt_spin_lock+0x38/0x110
2026-02-17T09:48:49.553336+01:00 lisa kernel: [ T1538] ? get_random_u16+0xc6/0x1c0
2026-02-17T09:48:49.553338+01:00 lisa kernel: [ T1538] ? srso_alias_return_thunk+0x5/0xfbef5
2026-02-17T09:48:49.553340+01:00 lisa kernel: [ T1538] ? rt_spin_lock+0x38/0x110
2026-02-17T09:48:49.553342+01:00 lisa kernel: [ T1538] ? srso_alias_return_thunk+0x5/0xfbef5
2026-02-17T09:48:49.553344+01:00 lisa kernel: [ T1538] ? rt_spin_unlock+0x5a/0xa0
2026-02-17T09:48:49.553346+01:00 lisa kernel: [ T1538] ? srso_alias_return_thunk+0x5/0xfbef5
2026-02-17T09:48:49.553347+01:00 lisa kernel: [ T1538] ? rt_spin_unlock+0x5a/0xa0
2026-02-17T09:48:49.553349+01:00 lisa kernel: [ T1538] ? srso_alias_return_thunk+0x5/0xfbef5
2026-02-17T09:48:49.553351+01:00 lisa kernel: [ T1538] ? __local_bh_enable_ip+0x73/0xa0
2026-02-17T09:48:49.553353+01:00 lisa kernel: [ T1538] ? srso_alias_return_thunk+0x5/0xfbef5
2026-02-17T09:48:49.553355+01:00 lisa kernel: [ T1538] __sys_sendmsg+0x68/0xc0
2026-02-17T09:48:49.553357+01:00 lisa kernel: [ T1538] do_syscall_64+0x65/0x2f0
2026-02-17T09:48:49.553359+01:00 lisa kernel: [ T1538] entry_SYSCALL_64_after_hwframe+0x55/0x5d
2026-02-17T09:48:49.553361+01:00 lisa kernel: [ T1538] RIP: 0033:0x7f3ae6d239ee
2026-02-17T09:48:49.553363+01:00 lisa kernel: [ T1538] Code: 08 0f 85 f5 4b ff ff 49 89 fb 48 89 f0 48 89 d7 48 89 ce 4c 89 c2 4d 89 ca 4c 8b 44 24 08 4c 8b 4c
24 10 4c 89 5c 24 08 0f 05 <c3> 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 80 00 00 00 00 48 83 ec 08
2026-02-17T09:48:49.553365+01:00 lisa kernel: [ T1538] RSP: 002b:00007f3ad75fd0a8 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
2026-02-17T09:48:49.553367+01:00 lisa kernel: [ T1538] RAX: ffffffffffffffda RBX: 00007f3ad75ff680 RCX: 00007f3ae6d239ee
2026-02-17T09:48:49.553367+01:00 lisa kernel: [ T1538] RDX: 0000000000000000 RSI: 00007f3ad75fd110 RDI: 0000000000000105
2026-02-17T09:48:49.553369+01:00 lisa kernel: [ T1538] RBP: 00007f3ad75fd110 R08: 0000000000000000 R09: 0000000000000000
2026-02-17T09:48:49.553371+01:00 lisa kernel: [ T1538] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
2026-02-17T09:48:49.553373+01:00 lisa kernel: [ T1538] R13: 00007f3ad75fd750 R14: 00007f3ad644c8d0 R15: 00007f3ad75fd750
2026-02-17T09:48:49.553375+01:00 lisa kernel: [ T1538] </TASK>
2026-02-17T09:48:49.553377+01:00 lisa kernel: [ T1538] ---[ end trace 0000000000000000 ]---
2026-02-17T09:48:49.553379+01:00 lisa kernel: [ T1538] Dead loop on virtual device wlp4s0, fix it urgently!
2026-02-17T09:48:49.553380+01:00 lisa kernel: [ T1538] __dev_queue_xmit 0: skb = ffff9ee68afd3b00 dev = wlp4s0
2026-02-17T09:48:49.553381+01:00 lisa kernel: [ T1538] __dev_queue_xmit 2: skb = ffff9ee68afd3b00 dev = wlp4s0 txq = ffff9ee6820b5a00
2026-02-17T09:48:49.553383+01:00 lisa kernel: [ T1538] __dev_queue_xmit: cpu = 7 xmit_lock_owner = 7
2026-02-17T09:48:49.553385+01:00 lisa kernel: [ T1538] Dead loop on virtual device wlp4s0, fix it urgently!
2026-02-17T09:48:49.553387+01:00 lisa kernel: [ T1521] __dev_queue_xmit 5: skb = 0000000000000000 dev = wlp4s0 txq = ffff9ee6820b5a00
Bert Karwatzki