syzbot

======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.0.60/6288 is trying to acquire lock:
ffff888033034750 (&mm->mmap_lock){++++}-{4:4}, at: mmap_read_lock_killable+0x1d/0x70 include/linux/mmap_lock.h:377

but task is already holding lock:
ffff88813fe34238 (&root->kernfs_rwsem){++++}-{4:4}, at: kernfs_fop_readdir+0x267/0x870 fs/kernfs/dir.c:1893

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #3 (&root->kernfs_rwsem){++++}-{4:4}:
       lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
       down_write+0x3a/0x50 kernel/locking/rwsem.c:1590
       kernfs_add_one+0x41/0x520 fs/kernfs/dir.c:791
       kernfs_create_dir_ns+0xde/0x130 fs/kernfs/dir.c:1093
       sysfs_create_dir_ns+0x123/0x280 fs/sysfs/dir.c:59
       create_dir lib/kobject.c:73 [inline]
       kobject_add_internal+0x5a5/0xb50 lib/kobject.c:240
       kobject_add_varg lib/kobject.c:374 [inline]
       kobject_add+0x155/0x220 lib/kobject.c:426
       btrfs_sysfs_add_block_group_type+0x244/0x6b0 fs/btrfs/sysfs.c:1908
       btrfs_create_pending_block_groups+0xb8f/0x1780 fs/btrfs/block-group.c:2818
       __btrfs_end_transaction+0x140/0x640 fs/btrfs/transaction.c:1071
       btrfs_inc_block_group_ro+0x641/0x6f0 fs/btrfs/block-group.c:3092
       btrfs_relocate_block_group+0x29d/0xba0 fs/btrfs/relocation.c:3936
       btrfs_relocate_chunk+0x12f/0x5d0 fs/btrfs/volumes.c:3451
       __btrfs_balance+0x186f/0x23f0 fs/btrfs/volumes.c:4227
       btrfs_balance+0xac2/0x11b0 fs/btrfs/volumes.c:4604
       btrfs_ioctl_balance+0x3d6/0x610 fs/btrfs/ioctl.c:3577
       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:63 [inline]
       do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #2 (btrfs_trans_num_extwriters){++++}-{0:0}:
       lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
       join_transaction+0x1a4/0xca0 fs/btrfs/transaction.c:321
       start_transaction+0x6b6/0x1620 fs/btrfs/transaction.c:705
       btrfs_rebuild_free_space_tree+0xad/0x6d0 fs/btrfs/free-space-tree.c:1340
       btrfs_start_pre_rw_mount+0x128f/0x1bf0 fs/btrfs/disk-io.c:3062
       open_ctree+0x2b11/0x3d20 fs/btrfs/disk-io.c:3619
       btrfs_fill_super fs/btrfs/super.c:987 [inline]
       btrfs_get_tree_super fs/btrfs/super.c:1951 [inline]
       btrfs_get_tree_subvol fs/btrfs/super.c:2088 [inline]
       btrfs_get_tree+0xf3c/0x1810 fs/btrfs/super.c:2122
       vfs_get_tree+0x92/0x2b0 fs/super.c:1751
       fc_mount fs/namespace.c:1208 [inline]
       do_new_mount_fc fs/namespace.c:3651 [inline]
       do_new_mount+0x302/0xa10 fs/namespace.c:3727
       do_mount fs/namespace.c:4050 [inline]
       __do_sys_mount fs/namespace.c:4238 [inline]
       __se_sys_mount+0x313/0x410 fs/namespace.c:4215
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #1 (btrfs_trans_num_writers){++++}-{0:0}:
       reacquire_held_locks+0x127/0x1d0 kernel/locking/lockdep.c:5385
       __lock_release kernel/locking/lockdep.c:5574 [inline]
       lock_release+0x1b4/0x3e0 kernel/locking/lockdep.c:5889
       percpu_up_read include/linux/percpu-rwsem.h:112 [inline]
       __sb_end_write include/linux/fs.h:1911 [inline]
       sb_end_intwrite+0x26/0x1c0 include/linux/fs.h:2028
       __btrfs_end_transaction+0x248/0x640 fs/btrfs/transaction.c:1076
       btrfs_dirty_inode+0x14c/0x190 fs/btrfs/inode.c:6284
       inode_update_time fs/inode.c:2117 [inline]
       touch_atime+0x2f9/0x6d0 fs/inode.c:2190
       file_accessed include/linux/fs.h:2673 [inline]
       btrfs_file_mmap_prepare+0xde/0x150 fs/btrfs/file.c:2049
       vfs_mmap_prepare include/linux/fs.h:2410 [inline]
       call_mmap_prepare mm/vma.c:2586 [inline]
       __mmap_region mm/vma.c:2654 [inline]
       mmap_region+0xb52/0x20f0 mm/vma.c:2740
       do_mmap+0xc23/0x10c0 mm/mmap.c:558
       vm_mmap_pgoff+0x2a9/0x4d0 mm/util.c:581
       ksys_mmap_pgoff+0x4e9/0x720 mm/mmap.c:604
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (&mm->mmap_lock){++++}-{4:4}:
       check_prev_add kernel/locking/lockdep.c:3165 [inline]
       check_prevs_add kernel/locking/lockdep.c:3284 [inline]
       validate_chain+0xb9b/0x2140 kernel/locking/lockdep.c:3908
       __lock_acquire+0xab9/0xd20 kernel/locking/lockdep.c:5237
       lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
       down_read_killable+0x9d/0x220 kernel/locking/rwsem.c:1560
       mmap_read_lock_killable+0x1d/0x70 include/linux/mmap_lock.h:377
       get_mmap_lock_carefully mm/mmap_lock.c:377 [inline]
       lock_mm_and_find_vma+0x2a8/0x300 mm/mmap_lock.c:428
       do_user_addr_fault+0x331/0x1380 arch/x86/mm/fault.c:1359
       handle_page_fault arch/x86/mm/fault.c:1476 [inline]
       exc_page_fault+0x82/0x100 arch/x86/mm/fault.c:1532
       asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
       user_access_begin arch/x86/include/asm/uaccess.h:-1 [inline]
       filldir+0x29c/0x6c0 fs/readdir.c:290
       dir_emit include/linux/fs.h:3986 [inline]
       kernfs_fop_readdir+0x53a/0x870 fs/kernfs/dir.c:1910
       iterate_dir+0x3a5/0x580 fs/readdir.c:108
       __do_sys_getdents fs/readdir.c:326 [inline]
       __se_sys_getdents+0xe4/0x250 fs/readdir.c:312
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xfa/0xfa0 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:
  &mm->mmap_lock --> btrfs_trans_num_extwriters --> &root->kernfs_rwsem

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  rlock(&root->kernfs_rwsem);
                               lock(btrfs_trans_num_extwriters);
                               lock(&root->kernfs_rwsem);
  rlock(&mm->mmap_lock);

 *** DEADLOCK ***

3 locks held by syz.0.60/6288:
 #0: ffff888029be4728 (&f->f_pos_lock){+.+.}-{4:4}, at: fdget_pos+0x253/0x320 fs/file.c:1232
 #1: ffff88805faf9778 (&type->i_mutex_dir_key#4){++++}-{4:4}, at: iterate_dir+0x29e/0x580 fs/readdir.c:101
 #2: ffff88813fe34238 (&root->kernfs_rwsem){++++}-{4:4}, at: kernfs_fop_readdir+0x267/0x870 fs/kernfs/dir.c:1893

stack backtrace:
CPU: 0 UID: 0 PID: 6288 Comm: syz.0.60 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
Call Trace:
 <TASK>
 dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
 print_circular_bug+0x2ee/0x310 kernel/locking/lockdep.c:2043
 check_noncircular+0x134/0x160 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+0xb9b/0x2140 kernel/locking/lockdep.c:3908
 __lock_acquire+0xab9/0xd20 kernel/locking/lockdep.c:5237
 lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
 down_read_killable+0x9d/0x220 kernel/locking/rwsem.c:1560
 mmap_read_lock_killable+0x1d/0x70 include/linux/mmap_lock.h:377
 get_mmap_lock_carefully mm/mmap_lock.c:377 [inline]
 lock_mm_and_find_vma+0x2a8/0x300 mm/mmap_lock.c:428
 do_user_addr_fault+0x331/0x1380 arch/x86/mm/fault.c:1359
 handle_page_fault arch/x86/mm/fault.c:1476 [inline]
 exc_page_fault+0x82/0x100 arch/x86/mm/fault.c:1532
 asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
RIP: 0010:filldir+0x29c/0x6c0 fs/readdir.c:294
Code: 8c ff 4c 89 ff 4c 89 e6 e8 41 7e 8c ff 4d 39 e7 0f 82 7c 02 00 00 49 39 ef 0f 87 73 02 00 00 0f 01 cb 0f ae e8 48 8b 44 24 50 <49> 89 44 24 08 48 8b 4c 24 08 48 8b 44 24 58 48 89 01 48 8b 04 24
RSP: 0018:ffffc9000956fc58 EFLAGS: 00050283
RAX: 000000002208a51e RBX: ffffc9000956fe38 RCX: ffff888025099e00
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000020
RBP: 00007ffffffff000 R08: 0000000000000000 R09: 0000000000000000
R10: dffffc0000000000 R11: ffffffff82328ef0 R12: 0000000000000000
R13: ffffffff8aea35e0 R14: 0000000000000006 R15: 0000000000000020
 dir_emit include/linux/fs.h:3986 [inline]
 kernfs_fop_readdir+0x53a/0x870 fs/kernfs/dir.c:1910
 iterate_dir+0x3a5/0x580 fs/readdir.c:108
 __do_sys_getdents fs/readdir.c:326 [inline]
 __se_sys_getdents+0xe4/0x250 fs/readdir.c:312
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f4b4086efc9
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 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 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f4b3eaad038 EFLAGS: 00000246 ORIG_RAX: 000000000000004e
RAX: ffffffffffffffda RBX: 00007f4b40ac6090 RCX: 00007f4b4086efc9
RDX: 0000000000001800 RSI: 0000000000000000 RDI: 0000000000000006
RBP: 00007f4b408f1f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f4b40ac6128 R14: 00007f4b40ac6090 R15: 00007ffc0650f5e8
 </TASK>
----------------
Code disassembly (best guess):
   0:	8c ff                	mov    %?,%edi
   2:	4c 89 ff             	mov    %r15,%rdi
   5:	4c 89 e6             	mov    %r12,%rsi
   8:	e8 41 7e 8c ff       	call   0xff8c7e4e
   d:	4d 39 e7             	cmp    %r12,%r15
  10:	0f 82 7c 02 00 00    	jb     0x292
  16:	49 39 ef             	cmp    %rbp,%r15
  19:	0f 87 73 02 00 00    	ja     0x292
  1f:	0f 01 cb             	stac
  22:	0f ae e8             	lfence
  25:	48 8b 44 24 50       	mov    0x50(%rsp),%rax
* 2a:	49 89 44 24 08       	mov    %rax,0x8(%r12) <-- trapping instruction
  2f:	48 8b 4c 24 08       	mov    0x8(%rsp),%rcx
  34:	48 8b 44 24 58       	mov    0x58(%rsp),%rax
  39:	48 89 01             	mov    %rax,(%rcx)
  3c:	48 8b 04 24          	mov    (%rsp),%rax