======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.0.2470/15056 is trying to acquire lock:
ffffffff8e99c6d0 (cgroup_threadgroup_rwsem){++++}-{0:0}, at: cgroup_procs_write_start+0x574/0x900

but task is already holding lock:
ffffffff8e7eb750 (cpu_hotplug_lock){++++}-{0:0}, at: cgroup_procs_write_start+0x568/0x900

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #8 (cpu_hotplug_lock){++++}-{0:0}:
       cpus_read_lock+0x42/0x160
       static_key_slow_inc+0x12/0x30
       setup_udp_tunnel_sock+0x2df/0x4f0
       l2tp_tunnel_register+0xe72/0x1570
       pppol2tp_connect+0x8b7/0x17c0
       __sys_connect+0x312/0x450
       __x64_sys_connect+0x7a/0x90
       do_syscall_64+0x15f/0xf80
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #7 (sk_lock-AF_INET6){+.+.}-{0:0}:
       lock_sock_nested+0x41/0x100
       inet_shutdown+0x6a/0x390
       nbd_mark_nsock_dead+0x2e9/0x560
       recv_work+0x1c2e/0x1d40
       process_scheduled_works+0xb5d/0x1860
       worker_thread+0xa53/0xfc0
       kthread+0x388/0x470
       ret_from_fork+0x514/0xb70
       ret_from_fork_asm+0x1a/0x30

-> #6 (&nsock->tx_lock){+.+.}-{4:4}:
       __mutex_lock+0x1a3/0x1550
       nbd_queue_rq+0x37b/0x1100
       blk_mq_dispatch_rq_list+0xa70/0x1910
       __blk_mq_sched_dispatch_requests+0xdcc/0x1600
       blk_mq_sched_dispatch_requests+0xd7/0x190
       blk_mq_run_work_fn+0x16c/0x300
       process_scheduled_works+0xb5d/0x1860
       worker_thread+0xa53/0xfc0
       kthread+0x388/0x470
       ret_from_fork+0x514/0xb70
       ret_from_fork_asm+0x1a/0x30

-> #5 (&cmd->lock){+.+.}-{4:4}:
       __mutex_lock+0x1a3/0x1550
       nbd_queue_rq+0xc6/0x1100
       blk_mq_dispatch_rq_list+0xa70/0x1910
       __blk_mq_sched_dispatch_requests+0xdcc/0x1600
       blk_mq_sched_dispatch_requests+0xd7/0x190
       blk_mq_run_work_fn+0x16c/0x300
       process_scheduled_works+0xb5d/0x1860
       worker_thread+0xa53/0xfc0
       kthread+0x388/0x470
       ret_from_fork+0x514/0xb70
       ret_from_fork_asm+0x1a/0x30

-> #4 (set->srcu){.+.+}-{0:0}:
       __synchronize_srcu+0xca/0x300
       elevator_switch+0x1e8/0x7a0
       elevator_change+0x2cc/0x450
       elevator_set_default+0x36c/0x430
       blk_register_queue+0x3e9/0x4e0
       __add_disk+0x677/0xd50
       add_disk_fwnode+0xfb/0x480
       nbd_dev_add+0x72c/0xb50
       nbd_init+0x168/0x1f0
       do_one_initcall+0x250/0x870
       do_initcall_level+0x104/0x190
       do_initcalls+0x59/0xa0
       kernel_init_freeable+0x2a6/0x3e0
       kernel_init+0x1d/0x1d0
       ret_from_fork+0x514/0xb70
       ret_from_fork_asm+0x1a/0x30

-> #3 (&q->elevator_lock){+.+.}-{4:4}:
       __mutex_lock+0x1a3/0x1550
       elevator_change+0x1b3/0x450
       elevator_set_none+0xb5/0x140
       blk_mq_update_nr_hw_queues+0x5e7/0x1a60
       nbd_start_device+0x17f/0xb10
       nbd_genl_connect+0x165b/0x1cf0
       genl_family_rcv_msg_doit+0x22a/0x330
       genl_rcv_msg+0x61c/0x7a0
       netlink_rcv_skb+0x232/0x4b0
       genl_rcv+0x28/0x40
       netlink_unicast+0x75c/0x8e0
       netlink_sendmsg+0x813/0xb40
       ____sys_sendmsg+0x972/0x9f0
       ___sys_sendmsg+0x2a5/0x360
       __x64_sys_sendmsg+0x1bd/0x2a0
       do_syscall_64+0x15f/0xf80
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #2 (&q->q_usage_counter(io)#50){++++}-{0:0}:
       blk_alloc_queue+0x546/0x680
       __blk_mq_alloc_disk+0x197/0x390
       nbd_dev_add+0x499/0xb50
       nbd_init+0x168/0x1f0
       do_one_initcall+0x250/0x870
       do_initcall_level+0x104/0x190
       do_initcalls+0x59/0xa0
       kernel_init_freeable+0x2a6/0x3e0
       kernel_init+0x1d/0x1d0
       ret_from_fork+0x514/0xb70
       ret_from_fork_asm+0x1a/0x30

-> #1 (fs_reclaim){+.+.}-{0:0}:
       fs_reclaim_acquire+0x71/0x100
       __kvmalloc_node_noprof+0x132/0x8a0
       futex_hash_allocate+0x448/0xe10
       copy_process+0x2aa7/0x4440
       kernel_clone+0x284/0x8f0
       __se_sys_clone3+0x33c/0x360
       do_syscall_64+0x15f/0xf80
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (cgroup_threadgroup_rwsem){++++}-{0:0}:
       __lock_acquire+0x15a5/0x2cf0
       lock_acquire+0x106/0x350
       percpu_down_write+0x54/0x320
       cgroup_procs_write_start+0x574/0x900
       __cgroup_procs_write+0x9b/0x300
       cgroup_procs_write+0x27/0x50
       cgroup_file_write+0x331/0x8f0
       kernfs_fop_write_iter+0x3af/0x540
       vfs_write+0x61d/0xb90
       ksys_write+0x150/0x270
       do_syscall_64+0x15f/0xf80
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

Chain exists of:
  cgroup_threadgroup_rwsem --> sk_lock-AF_INET6 --> cpu_hotplug_lock

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  rlock(cpu_hotplug_lock);
                               lock(sk_lock-AF_INET6);
                               lock(cpu_hotplug_lock);
  lock(cgroup_threadgroup_rwsem);

 *** DEADLOCK ***

5 locks held by syz.0.2470/15056:
 #0: ffff888109f668f0 (&f->f_pos_lock){+.+.}-{4:4}, at: fdget_pos+0x246/0x320
 #1: ffff888117e68410 (sb_writers#9){.+.+}-{0:0}, at: vfs_write+0x227/0xb90
 #2: ffff888111373080 (&of->mutex){+.+.}-{4:4}, at: kernfs_fop_write_iter+0x1de/0x540
 #3: ffffffff8e99c420 (cgroup_mutex){+.+.}-{4:4}, at: cgroup_kn_lock_live+0x13c/0x230
 #4: ffffffff8e7eb750 (cpu_hotplug_lock){++++}-{0:0}, at: cgroup_procs_write_start+0x568/0x900

stack backtrace:
CPU: 0 UID: 0 PID: 15056 Comm: syz.0.2470 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150
 print_circular_bug+0x2e1/0x300
 check_noncircular+0x12e/0x150
 __lock_acquire+0x15a5/0x2cf0
 lock_acquire+0x106/0x350
 percpu_down_write+0x54/0x320
 cgroup_procs_write_start+0x574/0x900
 __cgroup_procs_write+0x9b/0x300
 cgroup_procs_write+0x27/0x50
 cgroup_file_write+0x331/0x8f0
 kernfs_fop_write_iter+0x3af/0x540
 vfs_write+0x61d/0xb90
 ksys_write+0x150/0x270
 do_syscall_64+0x15f/0xf80
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f38af99cdd9
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:00007f38b0880028 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f38afc15fa0 RCX: 00007f38af99cdd9
RDX: 0000000000000012 RSI: 00002000000001c0 RDI: 0000000000000005
RBP: 00007f38afa32d69 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f38afc16038 R14: 00007f38afc15fa0 R15: 00007ffd7ebc5aa8
 </TASK>
