======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Tainted: G             L     
------------------------------------------------------
syz.6.4042/25828 is trying to acquire lock:
ffff88801c28d180 (&root->kernfs_rwsem){++++}-{4:4}, at: kernfs_remove_by_name_ns+0x4e/0x140 fs/kernfs/dir.c:1792

but task is already holding lock:
ffff88801c28d2a0 (&root->kernfs_supers_rwsem){++++}-{4:4}, at: kernfs_remove_by_name_ns+0x3f/0x140 fs/kernfs/dir.c:1791

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #9 (&root->kernfs_supers_rwsem){++++}-{4:4}:
       down_read+0x4a/0x330 kernel/locking/rwsem.c:1574
       kernfs_remove_by_name_ns+0x3f/0x140 fs/kernfs/dir.c:1791
       kernfs_remove_by_name include/linux/kernfs.h:644 [inline]
       remove_files fs/sysfs/group.c:28 [inline]
       sysfs_remove_group+0x101/0x2e0 fs/sysfs/group.c:328
       topology_remove_dev+0x22/0x30 drivers/base/topology.c:199
       cpuhp_invoke_callback+0x434/0x810 kernel/cpu.c:194
       __cpuhp_invoke_callback_range kernel/cpu.c:972 [inline]
       cpuhp_invoke_callback_range kernel/cpu.c:996 [inline]
       cpuhp_down_callbacks kernel/cpu.c:1386 [inline]
       _cpu_down+0x343/0x840 kernel/cpu.c:1457
       cpu_down_maps_locked kernel/cpu.c:1483 [inline]
       cpu_down kernel/cpu.c:1491 [inline]
       cpu_device_down+0x82/0xc0 kernel/cpu.c:1508
       device_offline+0x2d2/0x3c0 drivers/base/core.c:4279
       online_store+0x123/0x1a0 drivers/base/core.c:2879
       kernfs_fop_write_iter+0x3a4/0x540 fs/kernfs/file.c:345
       new_sync_write fs/read_write.c:595 [inline]
       vfs_write+0x612/0xba0 fs/read_write.c:687
       ksys_write+0x150/0x270 fs/read_write.c:739
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #8 (cpu_hotplug_lock){++++}-{0:0}:
       percpu_down_read_internal include/linux/percpu-rwsem.h:53 [inline]
       percpu_down_read include/linux/percpu-rwsem.h:77 [inline]
       cpus_read_lock+0x42/0x160 kernel/cpu.c:490
       static_key_slow_inc+0x12/0x30 kernel/jump_label.c:190
       udp_tunnel_encap_enable+0x86/0xb0 include/net/udp_tunnel.h:232
       udp_lib_setsockopt+0x5b6/0x6b0 net/ipv4/udp.c:2945
       udpv6_setsockopt+0x67/0xb0 net/ipv6/udp.c:1820
       do_sock_setsockopt+0x17c/0x1b0 net/socket.c:2368
       __sys_setsockopt net/socket.c:2393 [inline]
       __do_sys_setsockopt net/socket.c:2399 [inline]
       __se_sys_setsockopt net/socket.c:2396 [inline]
       __x64_sys_setsockopt+0x13d/0x1b0 net/socket.c:2396
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #7 (sk_lock-AF_INET6){+.+.}-{0:0}:
       lock_sock_nested+0x41/0x100 net/core/sock.c:3828
       lock_sock include/net/sock.h:1713 [inline]
       inet_shutdown+0x6a/0x390 net/ipv4/af_inet.c:913
       nbd_mark_nsock_dead+0x2cb/0x550 drivers/block/nbd.c:318
       recv_work+0x1cee/0x1e10 drivers/block/nbd.c:1021
       process_one_work kernel/workqueue.c:3322 [inline]
       process_scheduled_works+0xa8e/0x14e0 kernel/workqueue.c:3405
       worker_thread+0xa47/0xfb0 kernel/workqueue.c:3486
       kthread+0x388/0x470 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

-> #6 (&nsock->tx_lock){+.+.}-{4:4}:
       __mutex_lock_common kernel/locking/mutex.c:646 [inline]
       __mutex_lock+0x19d/0x1550 kernel/locking/mutex.c:821
       nbd_handle_cmd drivers/block/nbd.c:1143 [inline]
       nbd_queue_rq+0x373/0x1150 drivers/block/nbd.c:1207
       blk_mq_dispatch_rq_list+0x499/0x1990 block/blk-mq.c:2117
       __blk_mq_do_dispatch_sched block/blk-mq-sched.c:168 [inline]
       blk_mq_do_dispatch_sched block/blk-mq-sched.c:182 [inline]
       __blk_mq_sched_dispatch_requests+0xd36/0x1580 block/blk-mq-sched.c:307
       blk_mq_sched_dispatch_requests+0xd7/0x190 block/blk-mq-sched.c:329
       blk_mq_run_work_fn+0x16c/0x300 block/blk-mq.c:2532
       process_one_work kernel/workqueue.c:3322 [inline]
       process_scheduled_works+0xa8e/0x14e0 kernel/workqueue.c:3405
       worker_thread+0xa47/0xfb0 kernel/workqueue.c:3486
       kthread+0x388/0x470 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

-> #5 (&cmd->lock){+.+.}-{4:4}:
       __mutex_lock_common kernel/locking/mutex.c:646 [inline]
       __mutex_lock+0x19d/0x1550 kernel/locking/mutex.c:821
       nbd_queue_rq+0xc1/0x1150 drivers/block/nbd.c:1199
       blk_mq_dispatch_rq_list+0x499/0x1990 block/blk-mq.c:2117
       __blk_mq_do_dispatch_sched block/blk-mq-sched.c:168 [inline]
       blk_mq_do_dispatch_sched block/blk-mq-sched.c:182 [inline]
       __blk_mq_sched_dispatch_requests+0xd36/0x1580 block/blk-mq-sched.c:307
       blk_mq_sched_dispatch_requests+0xd7/0x190 block/blk-mq-sched.c:329
       blk_mq_run_work_fn+0x16c/0x300 block/blk-mq.c:2532
       process_one_work kernel/workqueue.c:3322 [inline]
       process_scheduled_works+0xa8e/0x14e0 kernel/workqueue.c:3405
       worker_thread+0xa47/0xfb0 kernel/workqueue.c:3486
       kthread+0x388/0x470 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

-> #4 (set->srcu){.+.+}-{0:0}:
       srcu_lock_sync include/linux/srcu.h:199 [inline]
       __synchronize_srcu+0xc9/0x2f0 kernel/rcu/srcutree.c:1481
       elevator_switch+0x1e8/0x7b0 block/elevator.c:576
       elevator_change+0x2fa/0x480 block/elevator.c:681
       elevator_set_default+0x375/0x440 block/elevator.c:754
       blk_register_queue+0x3f3/0x4e0 block/blk-sysfs.c:992
       __add_disk+0x6cb/0xe30 block/genhd.c:524
       add_disk_fwnode+0x100/0x3a0 block/genhd.c:593
       add_disk include/linux/blkdev.h:800 [inline]
       nbd_dev_add+0x733/0xb60 drivers/block/nbd.c:2021
       nbd_init+0x15f/0x1e0 drivers/block/nbd.c:2729
       do_one_initcall+0x250/0x870 init/main.c:1347
       do_initcall_level+0x10a/0x1a0 init/main.c:1409
       do_initcalls+0x59/0xa0 init/main.c:1425
       kernel_init_freeable+0x29d/0x3e0 init/main.c:1658
       kernel_init+0x1d/0x1d0 init/main.c:1548
       ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #3 (&q->elevator_lock){+.+.}-{4:4}:
       __mutex_lock_common kernel/locking/mutex.c:646 [inline]
       __mutex_lock+0x19d/0x1550 kernel/locking/mutex.c:821
       elevator_change+0x1af/0x480 block/elevator.c:679
       elevator_set_none+0xb5/0x140 block/elevator.c:769
       blk_mq_elv_switch_none block/blk-mq.c:5101 [inline]
       __blk_mq_update_nr_hw_queues block/blk-mq.c:5146 [inline]
       blk_mq_update_nr_hw_queues+0x5ef/0x19f0 block/blk-mq.c:5211
       nbd_start_device+0x189/0xb30 drivers/block/nbd.c:1526
       nbd_genl_connect+0x1597/0x1c10 drivers/block/nbd.c:2276
       genl_family_rcv_msg_doit+0x233/0x340 net/netlink/genetlink.c:1114
       genl_family_rcv_msg net/netlink/genetlink.c:1194 [inline]
       genl_rcv_msg+0x614/0x7a0 net/netlink/genetlink.c:1209
       netlink_rcv_skb+0x226/0x4a0 net/netlink/af_netlink.c:2556
       genl_rcv+0x28/0x40 net/netlink/genetlink.c:1218
       netlink_unicast_kernel net/netlink/af_netlink.c:1319 [inline]
       netlink_unicast+0x7bb/0x940 net/netlink/af_netlink.c:1345
       netlink_sendmsg+0x813/0xb40 net/netlink/af_netlink.c:1900
       sock_sendmsg_nosec+0x13a/0x180 net/socket.c:775
       __sock_sendmsg net/socket.c:790 [inline]
       ____sys_sendmsg+0x54e/0x850 net/socket.c:2684
       ___sys_sendmsg+0x2a5/0x360 net/socket.c:2738
       __sys_sendmsg net/socket.c:2770 [inline]
       __do_sys_sendmsg net/socket.c:2775 [inline]
       __se_sys_sendmsg net/socket.c:2773 [inline]
       __x64_sys_sendmsg+0x1b1/0x290 net/socket.c:2773
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #2 (&q->q_usage_counter(io)#50){++++}-{0:0}:
       blk_alloc_queue+0x544/0x690 block/blk-core.c:504
       blk_mq_alloc_queue block/blk-mq.c:4420 [inline]
       __blk_mq_alloc_disk+0x194/0x390 block/blk-mq.c:4467
       nbd_dev_add+0x494/0xb60 drivers/block/nbd.c:1991
       nbd_init+0x15f/0x1e0 drivers/block/nbd.c:2729
       do_one_initcall+0x250/0x870 init/main.c:1347
       do_initcall_level+0x10a/0x1a0 init/main.c:1409
       do_initcalls+0x59/0xa0 init/main.c:1425
       kernel_init_freeable+0x29d/0x3e0 init/main.c:1658
       kernel_init+0x1d/0x1d0 init/main.c:1548
       ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #1 (fs_reclaim){+.+.}-{0:0}:
       __fs_reclaim_acquire mm/page_alloc.c:4329 [inline]
       fs_reclaim_acquire+0x71/0x100 mm/page_alloc.c:4343
       might_alloc include/linux/sched/mm.h:317 [inline]
       slab_pre_alloc_hook mm/slub.c:4537 [inline]
       slab_alloc_node mm/slub.c:4897 [inline]
       kmem_cache_alloc_lru_noprof+0x65/0x5f0 mm/slub.c:4950
       alloc_inode+0xb8/0x1b0 fs/inode.c:340
       iget_locked+0x131/0x6a0 fs/inode.c:1477
       kernfs_get_inode+0x4f/0x770 fs/kernfs/inode.c:252
       kernfs_fill_super fs/kernfs/mount.c:308 [inline]
       kernfs_get_tree+0x5cd/0x980 fs/kernfs/mount.c:392
       vfs_get_tree+0x92/0x2a0 fs/super.c:1694
       fc_mount fs/namespace.c:1198 [inline]
       do_new_mount_fc fs/namespace.c:3765 [inline]
       do_new_mount+0x319/0xdc0 fs/namespace.c:3841
       do_mount fs/namespace.c:4174 [inline]
       __do_sys_mount fs/namespace.c:4390 [inline]
       __se_sys_mount+0x31d/0x420 fs/namespace.c:4367
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (&root->kernfs_rwsem){++++}-{4:4}:
       check_prev_add kernel/locking/lockdep.c:3165 [inline]
       check_prevs_add kernel/locking/lockdep.c:3284 [inline]
       validate_chain kernel/locking/lockdep.c:3908 [inline]
       __lock_acquire+0x1520/0x2cf0 kernel/locking/lockdep.c:5237
       lock_acquire+0x106/0x350 kernel/locking/lockdep.c:5868
       down_write+0x96/0x200 kernel/locking/rwsem.c:1631
       kernfs_remove_by_name_ns+0x4e/0x140 fs/kernfs/dir.c:1792
       kernfs_remove_by_name include/linux/kernfs.h:644 [inline]
       sysfs_unmerge_group+0xd3/0x140 fs/sysfs/group.c:408
       dpm_sysfs_remove+0x5f/0xd0 drivers/base/power/sysfs.c:830
       device_del+0x250/0x8f0 drivers/base/core.c:3936
       device_unregister drivers/base/core.c:4002 [inline]
       device_destroy+0xb2/0x1a0 drivers/base/core.c:4556
       tty_unregister_device+0x69/0x170 drivers/tty/tty_io.c:3307
       gser_free_inst+0x39/0x60 drivers/usb/gadget/function/f_serial.c:311
       usb_put_function_instance+0x90/0xb0 drivers/usb/gadget/functions.c:77
       config_item_cleanup fs/configfs/item.c:128 [inline]
       config_item_release+0x13a/0x2d0 fs/configfs/item.c:137
       configfs_rmdir+0x885/0x950 fs/configfs/dir.c:1571
       vfs_rmdir+0x3e6/0x6a0 fs/namei.c:5381
       filename_rmdir+0x292/0x520 fs/namei.c:5438
       __do_sys_rmdir fs/namei.c:5461 [inline]
       __se_sys_rmdir+0x2e/0x140 fs/namei.c:5458
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

Chain exists of:
  &root->kernfs_rwsem --> cpu_hotplug_lock --> &root->kernfs_supers_rwsem

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  rlock(&root->kernfs_supers_rwsem);
                               lock(cpu_hotplug_lock);
                               lock(&root->kernfs_supers_rwsem);
  lock(&root->kernfs_rwsem);

 *** DEADLOCK ***

4 locks held by syz.6.4042/25828:
 #0: ffff88801d32a450 (sb_writers#34){.+.+}-{0:0}, at: mnt_want_write+0x41/0x90 fs/namespace.c:494
 #1: ffff888071801900 (&default_group_class[depth - 1]#2/1){+.+.}-{4:4}, at: inode_lock_nested include/linux/fs.h:1069 [inline]
 #1: ffff888071801900 (&default_group_class[depth - 1]#2/1){+.+.}-{4:4}, at: __start_dirop fs/namei.c:2918 [inline]
 #1: ffff888071801900 (&default_group_class[depth - 1]#2/1){+.+.}-{4:4}, at: start_dirop fs/namei.c:2942 [inline]
 #1: ffff888071801900 (&default_group_class[depth - 1]#2/1){+.+.}-{4:4}, at: filename_rmdir+0x1cd/0x520 fs/namei.c:5431
 #2: ffff88802af61440 (&sb->s_type->i_mutex_key#43){++++}-{4:4}, at: inode_lock include/linux/fs.h:1024 [inline]
 #2: ffff88802af61440 (&sb->s_type->i_mutex_key#43){++++}-{4:4}, at: vfs_rmdir+0xf8/0x6a0 fs/namei.c:5366
 #3: ffff88801c28d2a0 (&root->kernfs_supers_rwsem){++++}-{4:4}, at: kernfs_remove_by_name_ns+0x3f/0x140 fs/kernfs/dir.c:1791

stack backtrace:
CPU: 0 UID: 0 PID: 25828 Comm: syz.6.4042 Tainted: G             L      syzkaller #0 PREEMPT(full) 
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/16/2026
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 print_circular_bug+0x2e1/0x300 kernel/locking/lockdep.c:2043
 check_noncircular+0x12e/0x150 kernel/locking/lockdep.c:2175
 check_prev_add kernel/locking/lockdep.c:3165 [inline]
 check_prevs_add kernel/locking/lockdep.c:3284 [inline]
 validate_chain kernel/locking/lockdep.c:3908 [inline]
 __lock_acquire+0x1520/0x2cf0 kernel/locking/lockdep.c:5237
 lock_acquire+0x106/0x350 kernel/locking/lockdep.c:5868
 down_write+0x96/0x200 kernel/locking/rwsem.c:1631
 kernfs_remove_by_name_ns+0x4e/0x140 fs/kernfs/dir.c:1792
 kernfs_remove_by_name include/linux/kernfs.h:644 [inline]
 sysfs_unmerge_group+0xd3/0x140 fs/sysfs/group.c:408
 dpm_sysfs_remove+0x5f/0xd0 drivers/base/power/sysfs.c:830
 device_del+0x250/0x8f0 drivers/base/core.c:3936
 device_unregister drivers/base/core.c:4002 [inline]
 device_destroy+0xb2/0x1a0 drivers/base/core.c:4556
 tty_unregister_device+0x69/0x170 drivers/tty/tty_io.c:3307
 gser_free_inst+0x39/0x60 drivers/usb/gadget/function/f_serial.c:311
 usb_put_function_instance+0x90/0xb0 drivers/usb/gadget/functions.c:77
 config_item_cleanup fs/configfs/item.c:128 [inline]
 config_item_release+0x13a/0x2d0 fs/configfs/item.c:137
 configfs_rmdir+0x885/0x950 fs/configfs/dir.c:1571
 vfs_rmdir+0x3e6/0x6a0 fs/namei.c:5381
 filename_rmdir+0x292/0x520 fs/namei.c:5438
 __do_sys_rmdir fs/namei.c:5461 [inline]
 __se_sys_rmdir+0x2e/0x140 fs/namei.c:5458
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f3b3019de99
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f3b31076028 EFLAGS: 00000246 ORIG_RAX: 0000000000000054
RAX: ffffffffffffffda RBX: 00007f3b30426180 RCX: 00007f3b3019de99
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000200000000000
RBP: 00007f3b30233eaf R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f3b30426218 R14: 00007f3b30426180 R15: 00007ffc8cc950a8
 </TASK>