======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.0.779/17578 is trying to acquire lock:
ffff88813ff6f238 (&root->kernfs_rwsem){++++}-{4:4}, at: kernfs_add_one+0x41/0x6f0 fs/kernfs/dir.c:825

but task is already holding lock:
ffff8880332d18c8 (&fs_devs->device_list_mutex){+.+.}-{4:4}, at: btrfs_init_new_device+0xa44/0x1b20 fs/btrfs/volumes.c:2996

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_common kernel/locking/rtmutex_api.c:559 [inline]
       mutex_lock_nested+0x5a/0x1d0 kernel/locking/rtmutex_api.c:578
       btrfs_init_new_device+0xa44/0x1b20 fs/btrfs/volumes.c:2996
       btrfs_ioctl_add_dev+0x1ad/0x640 fs/btrfs/ioctl.c:2556
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:597 [inline]
       __se_sys_ioctl+0xff/0x170 fs/ioctl.c:583
       do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
       do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #3 (btrfs_trans_num_extwriters){++++}-{0:0}:
       join_transaction+0x18a/0xc60 fs/btrfs/transaction.c:324
       start_transaction+0x6ed/0x17d0 fs/btrfs/transaction.c:718
       btrfs_create_common+0x15c/0x310 fs/btrfs/inode.c:6984
       btrfs_mkdir+0xc1/0xf0 fs/btrfs/inode.c:7129
       vfs_mkdir+0x408/0x620 fs/namei.c:5410
       filename_mkdirat+0x289/0x520 fs/namei.c:5443
       __do_sys_mkdirat fs/namei.c:5464 [inline]
       __se_sys_mkdirat+0x35/0x150 fs/namei.c:5461
       do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
       do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #2 (btrfs_trans_num_writers){++++}-{0:0}:
       __lock_release kernel/locking/lockdep.c:5593 [inline]
       lock_release+0x1a3/0x3c0 kernel/locking/lockdep.c:5930
       percpu_up_read include/linux/percpu-rwsem.h:114 [inline]
       __sb_end_write include/linux/fs/super.h:14 [inline]
       sb_end_intwrite+0x26/0x190 include/linux/fs/super.h:101
       __btrfs_end_transaction+0x23f/0x640 fs/btrfs/transaction.c:1095
       btrfs_dirty_inode+0x14c/0x190 fs/btrfs/inode.c:6460
       touch_atime+0x2f8/0x670 fs/inode.c:2326
       file_accessed include/linux/fs.h:2277 [inline]
       btrfs_file_mmap_prepare+0x189/0x200 fs/btrfs/file.c:2003
       vfs_mmap_prepare include/linux/fs.h:2069 [inline]
       call_mmap_prepare mm/vma.c:2776 [inline]
       __mmap_region mm/vma.c:2863 [inline]
       mmap_region+0xe9f/0x2240 mm/vma.c:2964
       do_mmap+0xe0e/0x1310 mm/mmap.c:573
       vm_mmap_pgoff+0x275/0x4e0 mm/util.c:581
       ksys_mmap_pgoff+0x4a6/0x720 mm/mmap.c:619
       do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
       do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #1 (&mm->mmap_lock){++++}-{4:4}:
       down_read_killable+0xa4/0x220 kernel/locking/rwsem.c:1599
       mmap_read_lock_killable+0x1d/0x70 include/linux/mmap_lock.h:609
       get_mmap_lock_carefully mm/mmap_lock.c:450 [inline]
       lock_mm_and_find_vma+0x2d6/0x340 mm/mmap_lock.c:501
       do_user_addr_fault+0x343/0x1340 arch/x86/mm/fault.c:1366
       handle_page_fault arch/x86/mm/fault.c:1483 [inline]
       exc_page_fault+0x6a/0xc0 arch/x86/mm/fault.c:1536
       asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:595
       filldir+0x2bd/0x630 fs/readdir.c:-1
       dir_emit include/linux/fs.h:3607 [inline]
       kernfs_fop_readdir+0x583/0x960 fs/kernfs/dir.c:1999
       iterate_dir+0x2f1/0x4e0 fs/readdir.c:110
       __do_sys_getdents fs/readdir.c:319 [inline]
       __se_sys_getdents+0xf1/0x270 fs/readdir.c:304
       do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
       do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (&root->kernfs_rwsem){++++}-{4:4}:
       check_prev_add kernel/locking/lockdep.c:3181 [inline]
       check_prevs_add kernel/locking/lockdep.c:3300 [inline]
       validate_chain kernel/locking/lockdep.c:3924 [inline]
       __lock_acquire+0x15ff/0x2e40 kernel/locking/lockdep.c:5254
       lock_acquire+0x115/0x350 kernel/locking/lockdep.c:5908
       down_write+0x3a/0x50 kernel/locking/rwsem.c:1631
       kernfs_add_one+0x41/0x6f0 fs/kernfs/dir.c:825
       kernfs_create_link+0x161/0x200 fs/kernfs/symlink.c:48
       sysfs_do_create_link_sd+0x83/0x110 fs/sysfs/symlink.c:44
       btrfs_sysfs_add_device+0x13f/0x650 fs/btrfs/sysfs.c:2155
       btrfs_init_new_device+0xea5/0x1b20 fs/btrfs/volumes.c:3038
       btrfs_ioctl_add_dev+0x1ad/0x640 fs/btrfs/ioctl.c:2556
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:597 [inline]
       __se_sys_ioctl+0xff/0x170 fs/ioctl.c:583
       do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
       do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
       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 ***

locks held by syz.0.779/17578: 4, last CPU#1:
 #0: ffff888040628800 (sb_internal#3){.+.+}-{0:0}, at: btrfs_init_new_device+0x736/0x1b20 fs/btrfs/volumes.c:2960
 #1: ffff888064d73190 (btrfs_trans_num_writers){++++}-{0:0}, at: join_transaction+0x14c/0xc60 fs/btrfs/transaction.c:323
 #2: ffff888064d731b8 (btrfs_trans_num_extwriters){++++}-{0:0}, at: join_transaction+0x14c/0xc60 fs/btrfs/transaction.c:323
 #3: ffff8880332d18c8 (&fs_devs->device_list_mutex){+.+.}-{4:4}, at: btrfs_init_new_device+0xa44/0x1b20 fs/btrfs/volumes.c:2996

stack backtrace:
CPU: 1 UID: 0 PID: 17578 Comm: syz.0.779 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 print_circular_bug+0x2e2/0x300 kernel/locking/lockdep.c:2059
 check_noncircular+0x12f/0x150 kernel/locking/lockdep.c:2191
 check_prev_add kernel/locking/lockdep.c:3181 [inline]
 check_prevs_add kernel/locking/lockdep.c:3300 [inline]
 validate_chain kernel/locking/lockdep.c:3924 [inline]
 __lock_acquire+0x15ff/0x2e40 kernel/locking/lockdep.c:5254
 lock_acquire+0x115/0x350 kernel/locking/lockdep.c:5908
 down_write+0x3a/0x50 kernel/locking/rwsem.c:1631
 kernfs_add_one+0x41/0x6f0 fs/kernfs/dir.c:825
 kernfs_create_link+0x161/0x200 fs/kernfs/symlink.c:48
 sysfs_do_create_link_sd+0x83/0x110 fs/sysfs/symlink.c:44
 btrfs_sysfs_add_device+0x13f/0x650 fs/btrfs/sysfs.c:2155
 btrfs_init_new_device+0xea5/0x1b20 fs/btrfs/volumes.c:3038
 btrfs_ioctl_add_dev+0x1ad/0x640 fs/btrfs/ioctl.c:2556
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl+0xff/0x170 fs/ioctl.c:583
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f9b2da1e159
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:00007f9b2bc55028 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f9b2dca6090 RCX: 00007f9b2da1e159
RDX: 0000200000001000 RSI: 000000005000940a RDI: 0000000000000007
RBP: 00007f9b2dab5024 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f9b2dca6128 R14: 00007f9b2dca6090 R15: 00007fffaf6702f8
 </TASK>