[syzbot] [openvswitch?] INFO: task hung in ovs_dp_masks_rebalance (8)

[syzbot] [openvswitch?] INFO: task hung in ovs_dp_masks_rebalance (8)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    a23cbb05744b Merge tag 'timers-urgent-2026-08-30' of git:/..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1107a379580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=5e4e3a0e188a497e
dashboard link: https://syzkaller.appspot.com/bug?extid=75ce0a9129024fa8cbf6
compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/eed599d932fb/disk-a23cbb05.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/74e8c268c325/vmlinux-a23cbb05.xz
kernel image: https://storage.googleapis.com/syzbot-assets/aa91aebf1baf/bzImage-a23cbb05.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+75ce0a9129024fa8cbf6@syzkaller.appspotmail.com

INFO: task kworker/0:0:9 blocked for more than 143 seconds.
      Not tainted syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:kworker/0:0     state:D stack:22128 pid:9     tgid:9     ppid:2      task_flags:0x4208060 flags:0x00080000
Workqueue: events ovs_dp_masks_rebalance
Call Trace:
 <TASK>
 context_switch kernel/sched/core.c:5520 [inline]
 __schedule+0x17db/0x58f0 kernel/sched/core.c:7270
 __schedule_loop kernel/sched/core.c:7347 [inline]
 schedule+0x164/0x2b0 kernel/sched/core.c:7362
 schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:7419
 __mutex_lock_common kernel/locking/mutex.c:726 [inline]
 __mutex_lock+0x7c1/0x1550 kernel/locking/mutex.c:821
 ovs_lock net/openvswitch/datapath.c:108 [inline]
 ovs_dp_masks_rebalance+0x2f/0xe0 net/openvswitch/datapath.c:2602
 process_one_work kernel/workqueue.c:3387 [inline]
 process_scheduled_works+0xc3d/0x1630 kernel/workqueue.c:3470
 worker_thread+0xa47/0xfb0 kernel/workqueue.c:3551
 kthread+0x38b/0x480 kernel/kthread.c:436
 ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>
INFO: task kworker/u8:1:13 blocked for more than 143 seconds.
      Not tainted syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:kworker/u8:1    state:D stack:22376 pid:13    tgid:13    ppid:2      task_flags:0x4208060 flags:0x00080000
Workqueue: ipv6_addrconf addrconf_verify_work
Call Trace:
 <TASK>
 context_switch kernel/sched/core.c:5520 [inline]
 __schedule+0x17db/0x58f0 kernel/sched/core.c:7270
 __schedule_loop kernel/sched/core.c:7347 [inline]
 schedule+0x164/0x2b0 kernel/sched/core.c:7362
 schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:7419
 __mutex_lock_common kernel/locking/mutex.c:726 [inline]
 __mutex_lock+0x7c1/0x1550 kernel/locking/mutex.c:821
 rtnl_net_lock include/linux/rtnetlink.h:134 [inline]
 addrconf_verify_work+0x19/0x30 net/ipv6/addrconf.c:4770
 process_one_work kernel/workqueue.c:3387 [inline]
 process_scheduled_works+0xc3d/0x1630 kernel/workqueue.c:3470
 worker_thread+0xa47/0xfb0 kernel/workqueue.c:3551
 kthread+0x38b/0x480 kernel/kthread.c:436
 ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>
INFO: task kworker/u8:2:27 blocked for more than 143 seconds.
      Not tainted syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:kworker/u8:2    state:D stack:23808 pid:27    tgid:27    ppid:2      task_flags:0x4208060 flags:0x00080000
Workqueue: netns cleanup_net
Call Trace:
 <TASK>
 context_switch kernel/sched/core.c:5520 [inline]
 __schedule+0x17db/0x58f0 kernel/sched/core.c:7270
 __schedule_loop kernel/sched/core.c:7347 [inline]
 schedule+0x164/0x2b0 kernel/sched/core.c:7362
 schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:7419
 __mutex_lock_common kernel/locking/mutex.c:726 [inline]
 __mutex_lock+0x7c1/0x1550 kernel/locking/mutex.c:821
 ops_exit_rtnl_list net/core/net_namespace.c:173 [inline]
 ops_undo_list+0x28e/0x8d0 net/core/net_namespace.c:249
 cleanup_net+0x572/0x810 net/core/net_namespace.c:706
 process_one_work kernel/workqueue.c:3387 [inline]
 process_scheduled_works+0xc3d/0x1630 kernel/workqueue.c:3470
 worker_thread+0xa47/0xfb0 kernel/workqueue.c:3551
 kthread+0x38b/0x480 kernel/kthread.c:436
 ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>
INFO: task kworker/1:1:31 blocked for more than 143 seconds.
      Not tainted syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:kworker/1:1     state:D stack:22232 pid:31    tgid:31    ppid:2      task_flags:0x4208060 flags:0x00080000
Workqueue: events ovs_dp_masks_rebalance
Call Trace:


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup

Re: [syzbot] [openvswitch?] INFO: task hung in ovs_dp_masks_rebalance (8)

[Ilya Maximets]

On 9/2/26 9:43 AM, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    a23cbb05744b Merge tag 'timers-urgent-2026-08-30' of git:/..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1107a379580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=5e4e3a0e188a497e
> dashboard link: https://syzkaller.appspot.com/bug?extid=75ce0a9129024fa8cbf6
> compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/eed599d932fb/disk-a23cbb05.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/74e8c268c325/vmlinux-a23cbb05.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/aa91aebf1baf/bzImage-a23cbb05.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+75ce0a9129024fa8cbf6@syzkaller.appspotmail.com
> 
> INFO: task kworker/0:0:9 blocked for more than 143 seconds.
>       Not tainted syzkaller #0
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> task:kworker/0:0     state:D stack:22128 pid:9     tgid:9     ppid:2      task_flags:0x4208060 flags:0x00080000
> Workqueue: events ovs_dp_masks_rebalance
> Call Trace:
>  <TASK>
>  context_switch kernel/sched/core.c:5520 [inline]
>  __schedule+0x17db/0x58f0 kernel/sched/core.c:7270
>  __schedule_loop kernel/sched/core.c:7347 [inline]
>  schedule+0x164/0x2b0 kernel/sched/core.c:7362
>  schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:7419
>  __mutex_lock_common kernel/locking/mutex.c:726 [inline]
>  __mutex_lock+0x7c1/0x1550 kernel/locking/mutex.c:821
>  ovs_lock net/openvswitch/datapath.c:108 [inline]
>  ovs_dp_masks_rebalance+0x2f/0xe0 net/openvswitch/datapath.c:2602
>  process_one_work kernel/workqueue.c:3387 [inline]
>  process_scheduled_works+0xc3d/0x1630 kernel/workqueue.c:3470
>  worker_thread+0xa47/0xfb0 kernel/workqueue.c:3551
>  kthread+0x38b/0x480 kernel/kthread.c:436
>  ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>  </TASK>
> INFO: task kworker/u8:1:13 blocked for more than 143 seconds.
>       Not tainted syzkaller #0
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> task:kworker/u8:1    state:D stack:22376 pid:13    tgid:13    ppid:2      task_flags:0x4208060 flags:0x00080000
> Workqueue: ipv6_addrconf addrconf_verify_work
> Call Trace:
>  <TASK>
>  context_switch kernel/sched/core.c:5520 [inline]
>  __schedule+0x17db/0x58f0 kernel/sched/core.c:7270
>  __schedule_loop kernel/sched/core.c:7347 [inline]
>  schedule+0x164/0x2b0 kernel/sched/core.c:7362
>  schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:7419
>  __mutex_lock_common kernel/locking/mutex.c:726 [inline]
>  __mutex_lock+0x7c1/0x1550 kernel/locking/mutex.c:821
>  rtnl_net_lock include/linux/rtnetlink.h:134 [inline]
>  addrconf_verify_work+0x19/0x30 net/ipv6/addrconf.c:4770
>  process_one_work kernel/workqueue.c:3387 [inline]
>  process_scheduled_works+0xc3d/0x1630 kernel/workqueue.c:3470
>  worker_thread+0xa47/0xfb0 kernel/workqueue.c:3551
>  kthread+0x38b/0x480 kernel/kthread.c:436
>  ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>  </TASK>
> INFO: task kworker/u8:2:27 blocked for more than 143 seconds.
>       Not tainted syzkaller #0
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> task:kworker/u8:2    state:D stack:23808 pid:27    tgid:27    ppid:2      task_flags:0x4208060 flags:0x00080000
> Workqueue: netns cleanup_net
> Call Trace:
>  <TASK>
>  context_switch kernel/sched/core.c:5520 [inline]
>  __schedule+0x17db/0x58f0 kernel/sched/core.c:7270
>  __schedule_loop kernel/sched/core.c:7347 [inline]
>  schedule+0x164/0x2b0 kernel/sched/core.c:7362
>  schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:7419
>  __mutex_lock_common kernel/locking/mutex.c:726 [inline]
>  __mutex_lock+0x7c1/0x1550 kernel/locking/mutex.c:821
>  ops_exit_rtnl_list net/core/net_namespace.c:173 [inline]
>  ops_undo_list+0x28e/0x8d0 net/core/net_namespace.c:249
>  cleanup_net+0x572/0x810 net/core/net_namespace.c:706
>  process_one_work kernel/workqueue.c:3387 [inline]
>  process_scheduled_works+0xc3d/0x1630 kernel/workqueue.c:3470
>  worker_thread+0xa47/0xfb0 kernel/workqueue.c:3551
>  kthread+0x38b/0x480 kernel/kthread.c:436
>  ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>  </TASK>
> INFO: task kworker/1:1:31 blocked for more than 143 seconds.
>       Not tainted syzkaller #0
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> task:kworker/1:1     state:D stack:22232 pid:31    tgid:31    ppid:2      task_flags:0x4208060 flags:0x00080000
> Workqueue: events ovs_dp_masks_rebalance
While this report points to ovs_dp_masks_rebalance, it seems like this job
is just a victim of something else getting stuck in the system.  My guess
is that something is holding rtnl blocking one of the OVS operations that
is also holding ovs_mutex and blcoks the rebalancing job from taking it.
But there is no enough information in this report to say what is actually
happening.  We'd need stack traces from all cores.

In the meantime, the patch I posted yesterday would actually reduce the
chance for ovs_dp_masks_rebalance to be flagged as a victim here, so maybe
we'll learn more in future reports:
  https://lore.kernel.org/r/20260902203200.1416026-1-i.maximets@ovn.org

Best regards, Ilya Maximets.