======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.9.3128/23600 is trying to acquire lock:
ffff8881012d4980 (&root->kernfs_rwsem){++++}-{4:4}, at: kernfs_add_one+0x41/0x5e0

but task is already holding lock:
ffff8881090d00d0 (&fs_devs->device_list_mutex){+.+.}-{4:4}, at: btrfs_init_new_device+0x9b4/0x19e0

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #4 (&fs_devs->device_list_mutex){+.+.}-{4:4}:
       __mutex_lock+0x19d/0x1550
       btrfs_create_pending_block_groups+0x743/0x15e0
       __btrfs_end_transaction+0x140/0x640
       flush_space+0x3e9/0xde0
       do_async_reclaim_data_space+0x137/0x550
       btrfs_async_reclaim_data_space+0x41/0x90
       process_scheduled_works+0xa8e/0x14e0
       worker_thread+0xa47/0xfb0
       kthread+0x388/0x470
       ret_from_fork+0x514/0xb70
       ret_from_fork_asm+0x1a/0x30

-> #3 (btrfs_trans_num_extwriters){++++}-{0:0}:
       join_transaction+0x18a/0xd40
       start_transaction+0x74d/0x18b0
       btrfs_create_common+0x15c/0x310
       btrfs_mkdir+0xc7/0xf0
       vfs_mkdir+0x406/0x620
       filename_mkdirat+0x285/0x510
       __se_sys_mkdirat+0x35/0x150
       do_syscall_64+0x174/0x580
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #2 (btrfs_trans_num_writers){++++}-{0:0}:
       lock_release+0x199/0x3c0
       sb_end_intwrite+0x26/0x190
       __btrfs_end_transaction+0x23f/0x640
       btrfs_dirty_inode+0x14c/0x190
       touch_atime+0x2f8/0x670
       btrfs_file_mmap_prepare+0x172/0x1f0
       mmap_region+0xe5a/0x2310
       do_mmap+0xc3b/0x10c0
       vm_mmap_pgoff+0x272/0x4e0
       ksys_mmap_pgoff+0x4dc/0x760
       do_syscall_64+0x174/0x580
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #1 (&mm->mmap_lock){++++}-{4:4}:
       down_read_killable+0x50/0x3b0
       mmap_read_lock_killable+0x1d/0x70
       lock_mm_and_find_vma+0x2d6/0x340
       do_user_addr_fault+0x341/0x1340
       exc_page_fault+0x6a/0xc0
       asm_exc_page_fault+0x26/0x30
       filldir64+0x2d4/0x620
       kernfs_fop_readdir+0x580/0x960
       iterate_dir+0x2e2/0x4d0
       __se_sys_getdents64+0xf1/0x280
       do_syscall_64+0x174/0x580
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (&root->kernfs_rwsem){++++}-{4:4}:
       __lock_acquire+0x1520/0x2cf0
       lock_acquire+0x106/0x350
       down_write+0x96/0x200
       kernfs_add_one+0x41/0x5e0
       kernfs_create_link+0x161/0x200
       sysfs_do_create_link_sd+0x83/0x110
       btrfs_sysfs_add_device+0x13f/0x650
       btrfs_init_new_device+0xdb8/0x19e0
       btrfs_ioctl_add_dev+0x1ad/0x640
       __se_sys_ioctl+0xfc/0x170
       do_syscall_64+0x174/0x580
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

Chain exists of:
  &root->kernfs_rwsem --> btrfs_trans_num_extwriters --> &fs_devs->device_list_mutex

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&fs_devs->device_list_mutex);
                               lock(btrfs_trans_num_extwriters);
                               lock(&fs_devs->device_list_mutex);
  lock(&root->kernfs_rwsem);

 *** DEADLOCK ***

4 locks held by syz.9.3128/23600:
 #0: ffff88810f770640 (sb_internal#4){.+.+}-{0:0}, at: btrfs_init_new_device+0x699/0x19e0
 #1: ffff888112e92688 (btrfs_trans_num_writers){++++}-{0:0}, at: join_transaction+0x14c/0xd40
 #2: ffff888112e926b0 (btrfs_trans_num_extwriters){++++}-{0:0}, at: join_transaction+0x14c/0xd40
 #3: ffff8881090d00d0 (&fs_devs->device_list_mutex){+.+.}-{4:4}, at: btrfs_init_new_device+0x9b4/0x19e0

stack backtrace:
CPU: 0 UID: 0 PID: 23600 Comm: syz.9.3128 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+0x1520/0x2cf0
 lock_acquire+0x106/0x350
 down_write+0x96/0x200
 kernfs_add_one+0x41/0x5e0
 kernfs_create_link+0x161/0x200
 sysfs_do_create_link_sd+0x83/0x110
 btrfs_sysfs_add_device+0x13f/0x650
 btrfs_init_new_device+0xdb8/0x19e0
 btrfs_ioctl_add_dev+0x1ad/0x640
 __se_sys_ioctl+0xfc/0x170
 do_syscall_64+0x174/0x580
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f2d3a39e0d9
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:00007f2d3b22e028 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f2d3a625fa0 RCX: 00007f2d3a39e0d9
RDX: 0000200000001000 RSI: 000000005000940a RDI: 0000000000000007
RBP: 00007f2d3a435024 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f2d3a626038 R14: 00007f2d3a625fa0 R15: 00007ffedc9230d8
 </TASK>
BTRFS info (device loop9): disk added /dev/nullb0
