On 7/2/25 12:37 PM, Zigit Zo wrote:

This bug happens if the VMM sends a VIRTIO_NET_S_ANNOUNCE request while
the virtio-net driver is still probing with rtnl_lock() hold, this will
cause a recursive mutex in netdev_notify_peers().

Fix it by temporarily save the announce status while probing, and then in
virtnet_open(), if it sees a delayed announce work is there, it starts to
schedule the virtnet_config_changed_work().

Another possible solution is to directly check whether rtnl_is_locked()
and call __netdev_notify_peers(), but in that way means we need to relies
on netdev_queue to schedule the arp packets after ndo_open(), which we
thought is not very intuitive.

We've observed a softlockup with Ubuntu 24.04, and can be reproduced with
QEMU sending the announce_self rapidly while booting.

[  494.167473] INFO: task swapper/0:1 blocked for more than 368 seconds.
[  494.167667]       Not tainted 6.8.0-57-generic #59-Ubuntu
[  494.167810] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[  494.168015] task:swapper/0       state:D stack:0     pid:1     tgid:1     ppid:0      flags:0x00004000
[  494.168260] Call Trace:
[  494.168329]  <TASK>
[  494.168389]  __schedule+0x27c/0x6b0
[  494.168495]  schedule+0x33/0x110
[  494.168585]  schedule_preempt_disabled+0x15/0x30
[  494.168709]  __mutex_lock.constprop.0+0x42f/0x740
[  494.168835]  __mutex_lock_slowpath+0x13/0x20
[  494.168949]  mutex_lock+0x3c/0x50
[  494.169039]  rtnl_lock+0x15/0x20
[  494.169128]  netdev_notify_peers+0x12/0x30
[  494.169240]  virtnet_config_changed_work+0x152/0x1a0
[  494.169377]  virtnet_probe+0xa48/0xe00
[  494.169484]  ? vp_get+0x4d/0x100
[  494.169574]  virtio_dev_probe+0x1e9/0x310
[  494.169682]  really_probe+0x1c7/0x410
[  494.169783]  __driver_probe_device+0x8c/0x180
[  494.169901]  driver_probe_device+0x24/0xd0
[  494.170011]  __driver_attach+0x10b/0x210
[  494.170117]  ? __pfx___driver_attach+0x10/0x10
[  494.170237]  bus_for_each_dev+0x8d/0xf0
[  494.170341]  driver_attach+0x1e/0x30
[  494.170440]  bus_add_driver+0x14e/0x290
[  494.170548]  driver_register+0x5e/0x130
[  494.170651]  ? __pfx_virtio_net_driver_init+0x10/0x10
[  494.170788]  register_virtio_driver+0x20/0x40
[  494.170905]  virtio_net_driver_init+0x97/0xb0
[  494.171022]  do_one_initcall+0x5e/0x340
[  494.171128]  do_initcalls+0x107/0x230
[  494.171228]  ? __pfx_kernel_init+0x10/0x10
[  494.171340]  kernel_init_freeable+0x134/0x210
[  494.171462]  kernel_init+0x1b/0x200
[  494.171560]  ret_from_fork+0x47/0x70
[  494.171659]  ? __pfx_kernel_init+0x10/0x10
[  494.171769]  ret_from_fork_asm+0x1b/0x30
[  494.171875]  </TASK>

Fixes: df28de7b0050 ("virtio-net: synchronize operstate with admin state on up/down")
Signed-off-by: Zigit Zo <redacted>

@Micheal: I think this addresses your concerns on v1, WDYT?

/P