loop2: detected capacity change from 0 to 1024
EXT4-fs (loop2): mounted filesystem 00000000-0000-0000-0000-000000000000 r/w without journal. Quota mode: writeback.
======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.2.2848/25653 is trying to acquire lock:
ffff888023cad7e0 (&mm->mmap_lock){++++}-{4:4}, at: mmap_read_lock_killable+0x1d/0x70

but task is already holding lock:
ffff8881137ef6a0 (&type->i_mutex_dir_key#3){++++}-{4:4}, at: iterate_dir+0x292/0x570

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #4 (&type->i_mutex_dir_key#3){++++}-{4:4}:
       lock_acquire+0x120/0x360
       down_read+0x46/0x2e0
       lookup_slow+0x46/0x70
       walk_component+0x2d2/0x400
       path_lookupat+0x163/0x430
       filename_lookup+0x212/0x570
       kern_path+0x35/0x50
       device_list_add+0xe2a/0x22a0
       btrfs_scan_one_device+0x3ee/0x650
       btrfs_control_ioctl+0x11f/0x360
       __se_sys_ioctl+0xfc/0x170
       do_syscall_64+0xfa/0x3b0
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #3 (&fs_devs->device_list_mutex){+.+.}-{4:4}:
       lock_acquire+0x120/0x360
       __mutex_lock+0x187/0x1350
       btrfs_create_pending_block_groups+0x5f8/0x1780
       __btrfs_end_transaction+0x140/0x640
       flush_space+0x315/0xcd0
       do_async_reclaim_metadata_space+0x144/0x390
       btrfs_async_reclaim_metadata_space+0x89/0xe0
       process_scheduled_works+0xae1/0x17b0
       worker_thread+0x8a0/0xda0
       kthread+0x711/0x8a0
       ret_from_fork+0x439/0x7d0
       ret_from_fork_asm+0x1a/0x30

-> #2 (btrfs_trans_num_extwriters){++++}-{0:0}:
       lock_acquire+0x120/0x360
       join_transaction+0x1a4/0xd70
       start_transaction+0x6b6/0x1620
       btrfs_rebuild_free_space_tree+0xad/0x760
       btrfs_start_pre_rw_mount+0x128f/0x1bf0
       open_ctree+0x293a/0x3a10
       btrfs_get_tree+0xf55/0x1820
       vfs_get_tree+0x92/0x2b0
       do_new_mount+0x2a2/0x9e0
       __se_sys_mount+0x317/0x410
       do_syscall_64+0xfa/0x3b0
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #1 (btrfs_trans_num_writers){++++}-{0:0}:
       reacquire_held_locks+0x127/0x1d0
       lock_release+0x1b4/0x3e0
       sb_end_intwrite+0x26/0x1c0
       __btrfs_end_transaction+0x248/0x640
       btrfs_dirty_inode+0x14c/0x190
       touch_atime+0x2f9/0x6d0
       btrfs_file_mmap_prepare+0xd5/0x140
       mmap_region+0xb41/0x20c0
       do_mmap+0xc45/0x10d0
       vm_mmap_pgoff+0x2a6/0x4d0
       ksys_mmap_pgoff+0x51f/0x760
       do_syscall_64+0xfa/0x3b0
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (&mm->mmap_lock){++++}-{4:4}:
       validate_chain+0xb9b/0x2140
       __lock_acquire+0xab9/0xd20
       lock_acquire+0x120/0x360
       down_read_killable+0x50/0x350
       mmap_read_lock_killable+0x1d/0x70
       lock_mm_and_find_vma+0x2a8/0x300
       do_user_addr_fault+0x331/0x1390
       exc_page_fault+0x76/0xf0
       asm_exc_page_fault+0x26/0x30
       filldir+0x2a2/0x6c0
       ext4_readdir+0x327e/0x3b60
       iterate_dir+0x399/0x570
       __se_sys_getdents+0xe4/0x250
       do_syscall_64+0xfa/0x3b0
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

Chain exists of:
  &mm->mmap_lock --> &fs_devs->device_list_mutex --> &type->i_mutex_dir_key#3

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  rlock(&type->i_mutex_dir_key#3);
                               lock(&fs_devs->device_list_mutex);
                               lock(&type->i_mutex_dir_key#3);
  rlock(&mm->mmap_lock);

 *** DEADLOCK ***

2 locks held by syz.2.2848/25653:
 #0: ffff8881104adef8 (&f->f_pos_lock){+.+.}-{4:4}, at: fdget_pos+0x247/0x320
 #1: ffff8881137ef6a0 (&type->i_mutex_dir_key#3){++++}-{4:4}, at: iterate_dir+0x292/0x570

stack backtrace:
CPU: 1 UID: 0 PID: 25653 Comm: syz.2.2848 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+0x189/0x250
 print_circular_bug+0x2ee/0x310
 check_noncircular+0x134/0x160
 validate_chain+0xb9b/0x2140
 __lock_acquire+0xab9/0xd20
 lock_acquire+0x120/0x360
 down_read_killable+0x50/0x350
 mmap_read_lock_killable+0x1d/0x70
 lock_mm_and_find_vma+0x2a8/0x300
 do_user_addr_fault+0x331/0x1390
 exc_page_fault+0x76/0xf0
 asm_exc_page_fault+0x26/0x30
RIP: 0010:filldir+0x2a2/0x6c0
Code: 86 ff 4c 89 ff 4c 89 e6 e8 5b 42 86 ff 4d 39 e7 0f 82 7c 02 00 00 49 39 ef 0f 87 73 02 00 00 0f 1f 00 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:ffffc9000393fa48 EFLAGS: 00010283
RAX: 07953df87dca8abe RBX: ffffc9000393fe38 RCX: ffff888020525640
RDX: 0000000000000000 RSI: 0000200000001fc0 RDI: 0000200000001fe0
RBP: 00007ffffffff000 R08: 000000000000000c R09: 0000000000000004
R10: dffffc0000000000 R11: ffffffff82397ae0 R12: 0000200000001fc0
R13: ffff88810f8a3cae R14: 0000000000000005 R15: 0000200000001fe0
 ext4_readdir+0x327e/0x3b60
 iterate_dir+0x399/0x570
 __se_sys_getdents+0xe4/0x250
 do_syscall_64+0xfa/0x3b0
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f6765f8eba9
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:00007f6766d44038 EFLAGS: 00000246 ORIG_RAX: 000000000000004e
RAX: ffffffffffffffda RBX: 00007f67661d5fa0 RCX: 00007f6765f8eba9
RDX: 0000000020002078 RSI: 0000200000001fc0 RDI: 0000000000000005
RBP: 00007f6766011e19 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f67661d6038 R14: 00007f67661d5fa0 R15: 00007fffdd5fc9b8
 </TASK>
