syzbot

loop5: detected capacity change from 0 to 32768
======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.5.749/8429 is trying to acquire lock:
ffff0000c09208e0 (&type->i_mutex_dir_key#2){++++}-{3:3}, at: inode_lock_shared include/linux/fs.h:768 [inline]
ffff0000c09208e0 (&type->i_mutex_dir_key#2){++++}-{3:3}, at: lookup_slow+0x4c/0x80 fs/namei.c:1714

but task is already holding lock:
ffff0000f59464e0 (&fs_devs->device_list_mutex){+.+.}-{3:3}, at: device_list_add+0x7f4/0x1c84 fs/btrfs/volumes.c:860

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #6 (&fs_devs->device_list_mutex){+.+.}-{3:3}:
       __mutex_lock_common+0x190/0x1f60 kernel/locking/mutex.c:603
       __mutex_lock kernel/locking/mutex.c:747 [inline]
       mutex_lock_nested+0x38/0x44 kernel/locking/mutex.c:799
       btrfs_run_dev_stats+0xf8/0xe4c fs/btrfs/volumes.c:8071
       commit_cowonly_roots+0x170/0x71c fs/btrfs/transaction.c:1298
       btrfs_commit_transaction+0x10e8/0x2884 fs/btrfs/transaction.c:2428
       btrfs_sync_fs+0x530/0xa28 fs/btrfs/super.c:1535
       sync_filesystem+0x1a0/0x218 fs/sync.c:66
       generic_shutdown_super+0x70/0x324 fs/super.c:474
       kill_anon_super+0x4c/0x80 fs/super.c:1128
       btrfs_kill_super+0x40/0x58 fs/btrfs/super.c:2448
       deactivate_locked_super+0xac/0x120 fs/super.c:332
       deactivate_super+0xe4/0x104 fs/super.c:363
       cleanup_mnt+0x390/0x418 fs/namespace.c:1191
       __cleanup_mnt+0x20/0x30 fs/namespace.c:1198
       task_work_run+0x1ec/0x278 kernel/task_work.c:203
       resume_user_mode_work include/linux/resume_user_mode.h:49 [inline]
       do_notify_resume+0x1fa0/0x2aa4 arch/arm64/kernel/signal.c:1151
       prepare_exit_to_user_mode arch/arm64/kernel/entry-common.c:137 [inline]
       exit_to_user_mode arch/arm64/kernel/entry-common.c:142 [inline]
       el0_svc+0x98/0x128 arch/arm64/kernel/entry-common.c:638
       el0t_64_sync_handler+0x84/0xf0 arch/arm64/kernel/entry-common.c:655
       el0t_64_sync+0x18c/0x190 arch/arm64/kernel/entry.S:585

-> #5 (&fs_info->reloc_mutex){+.+.}-{3:3}:
       __mutex_lock_common+0x190/0x1f60 kernel/locking/mutex.c:603
       __mutex_lock kernel/locking/mutex.c:747 [inline]
       mutex_lock_nested+0x38/0x44 kernel/locking/mutex.c:799
       btrfs_record_root_in_trans fs/btrfs/transaction.c:488 [inline]
       start_transaction+0x434/0x1050 fs/btrfs/transaction.c:731
       btrfs_join_transaction+0x30/0x40 fs/btrfs/transaction.c:781
       btrfs_dirty_inode+0xb4/0x1c0 fs/btrfs/inode.c:6281
       btrfs_update_time+0x170/0x18c fs/btrfs/inode.c:6323
       inode_update_time fs/inode.c:1958 [inline]
       __file_update_time+0xc0/0x21c fs/inode.c:2147
       file_update_time+0x270/0x2ec fs/inode.c:2178
       btrfs_page_mkwrite+0x39c/0xb88 fs/btrfs/inode.c:8606
       do_page_mkwrite+0x13c/0x358 mm/memory.c:3009
       wp_page_shared+0x148/0x540 mm/memory.c:3358
       do_wp_page+0xca0/0xeec mm/memory.c:3508
       handle_pte_fault mm/memory.c:5047 [inline]
       __handle_mm_fault mm/memory.c:5171 [inline]
       handle_mm_fault+0x1514/0x2fb0 mm/memory.c:5292
       __do_page_fault arch/arm64/mm/fault.c:499 [inline]
       do_page_fault+0x310/0x98c arch/arm64/mm/fault.c:583
       do_mem_abort+0x70/0x194 arch/arm64/mm/fault.c:804
       el0_da+0x70/0x144 arch/arm64/kernel/entry-common.c:515
       el0t_64_sync_handler+0x90/0xf0 arch/arm64/kernel/entry-common.c:658
       el0t_64_sync+0x18c/0x190 arch/arm64/kernel/entry.S:585

-> #4 (btrfs_trans_num_extwriters){++++}-{0:0}:
       join_transaction+0x180/0xb68 fs/btrfs/transaction.c:303
       start_transaction+0x858/0x1050 fs/btrfs/transaction.c:668
       btrfs_join_transaction+0x30/0x40 fs/btrfs/transaction.c:781
       btrfs_dirty_inode+0xb4/0x1c0 fs/btrfs/inode.c:6281
       btrfs_update_time+0x170/0x18c fs/btrfs/inode.c:6323
       inode_update_time fs/inode.c:1958 [inline]
       __file_update_time+0xc0/0x21c fs/inode.c:2147
       file_update_time+0x270/0x2ec fs/inode.c:2178
       btrfs_page_mkwrite+0x39c/0xb88 fs/btrfs/inode.c:8606
       do_page_mkwrite+0x13c/0x358 mm/memory.c:3009
       wp_page_shared+0x148/0x540 mm/memory.c:3358
       do_wp_page+0xca0/0xeec mm/memory.c:3508
       handle_pte_fault mm/memory.c:5047 [inline]
       __handle_mm_fault mm/memory.c:5171 [inline]
       handle_mm_fault+0x1514/0x2fb0 mm/memory.c:5292
       __do_page_fault arch/arm64/mm/fault.c:499 [inline]
       do_page_fault+0x310/0x98c arch/arm64/mm/fault.c:583
       do_mem_abort+0x70/0x194 arch/arm64/mm/fault.c:804
       el0_da+0x70/0x144 arch/arm64/kernel/entry-common.c:515
       el0t_64_sync_handler+0x90/0xf0 arch/arm64/kernel/entry-common.c:658
       el0t_64_sync+0x18c/0x190 arch/arm64/kernel/entry.S:585

-> #3 (btrfs_trans_num_writers){++++}-{0:0}:
       __lock_release kernel/locking/lockdep.c:5376 [inline]
       lock_release+0x308/0x908 kernel/locking/lockdep.c:5682
       percpu_up_read include/linux/percpu-rwsem.h:99 [inline]
       __sb_end_write include/linux/fs.h:1886 [inline]
       sb_end_intwrite+0x54/0x364 include/linux/fs.h:1942
       __btrfs_end_transaction+0x1f4/0x5f0 fs/btrfs/transaction.c:1018
       btrfs_end_transaction+0x24/0x34 fs/btrfs/transaction.c:1052
       btrfs_dirty_inode+0x16c/0x1c0 fs/btrfs/inode.c:6295
       btrfs_update_time+0x170/0x18c fs/btrfs/inode.c:6323
       inode_update_time fs/inode.c:1958 [inline]
       __file_update_time+0xc0/0x21c fs/inode.c:2147
       file_update_time+0x270/0x2ec fs/inode.c:2178
       btrfs_page_mkwrite+0x39c/0xb88 fs/btrfs/inode.c:8606
       do_page_mkwrite+0x13c/0x358 mm/memory.c:3009
       wp_page_shared+0x148/0x540 mm/memory.c:3358
       do_wp_page+0xca0/0xeec mm/memory.c:3508
       handle_pte_fault mm/memory.c:5047 [inline]
       __handle_mm_fault mm/memory.c:5171 [inline]
       handle_mm_fault+0x1514/0x2fb0 mm/memory.c:5292
       __do_page_fault arch/arm64/mm/fault.c:499 [inline]
       do_page_fault+0x310/0x98c arch/arm64/mm/fault.c:583
       do_mem_abort+0x70/0x194 arch/arm64/mm/fault.c:804
       el0_da+0x70/0x144 arch/arm64/kernel/entry-common.c:515
       el0t_64_sync_handler+0x90/0xf0 arch/arm64/kernel/entry-common.c:658
       el0t_64_sync+0x18c/0x190 arch/arm64/kernel/entry.S:585

-> #2 (sb_pagefaults#2){.+.+}-{0:0}:
       percpu_down_read include/linux/percpu-rwsem.h:51 [inline]
       __sb_start_write include/linux/fs.h:1891 [inline]
       sb_start_pagefault include/linux/fs.h:1995 [inline]
       btrfs_page_mkwrite+0x198/0xb88 fs/btrfs/inode.c:8590
       do_page_mkwrite+0x13c/0x358 mm/memory.c:3009
       wp_page_shared+0x148/0x540 mm/memory.c:3358
       do_wp_page+0xca0/0xeec mm/memory.c:3508
       handle_pte_fault mm/memory.c:5047 [inline]
       __handle_mm_fault mm/memory.c:5171 [inline]
       handle_mm_fault+0x1514/0x2fb0 mm/memory.c:5292
       __do_page_fault arch/arm64/mm/fault.c:499 [inline]
       do_page_fault+0x310/0x98c arch/arm64/mm/fault.c:583
       do_mem_abort+0x70/0x194 arch/arm64/mm/fault.c:804
       el0_da+0x70/0x144 arch/arm64/kernel/entry-common.c:515
       el0t_64_sync_handler+0x90/0xf0 arch/arm64/kernel/entry-common.c:658
       el0t_64_sync+0x18c/0x190 arch/arm64/kernel/entry.S:585

-> #1 (&mm->mmap_lock){++++}-{3:3}:
       __might_fault+0xc4/0x124 mm/memory.c:5850
       filldir64+0x2cc/0x8fc fs/readdir.c:335
       dir_emit_dot include/linux/fs.h:3698 [inline]
       dir_emit_dots include/linux/fs.h:3709 [inline]
       dcache_readdir+0x140/0x62c fs/libfs.c:197
       iterate_dir+0x1f0/0x4cc fs/readdir.c:-1
       __do_sys_getdents64 fs/readdir.c:369 [inline]
       __se_sys_getdents64 fs/readdir.c:354 [inline]
       __arm64_sys_getdents64+0x11c/0x320 fs/readdir.c:354
       __invoke_syscall arch/arm64/kernel/syscall.c:38 [inline]
       invoke_syscall+0x98/0x2b4 arch/arm64/kernel/syscall.c:52
       el0_svc_common+0x138/0x258 arch/arm64/kernel/syscall.c:140
       do_el0_svc+0x58/0x130 arch/arm64/kernel/syscall.c:204
       el0_svc+0x58/0x128 arch/arm64/kernel/entry-common.c:637
       el0t_64_sync_handler+0x84/0xf0 arch/arm64/kernel/entry-common.c:655
       el0t_64_sync+0x18c/0x190 arch/arm64/kernel/entry.S:585

-> #0 (&type->i_mutex_dir_key#2){++++}-{3:3}:
       check_prev_add kernel/locking/lockdep.c:3090 [inline]
       check_prevs_add kernel/locking/lockdep.c:3209 [inline]
       validate_chain kernel/locking/lockdep.c:3825 [inline]
       __lock_acquire+0x2880/0x6800 kernel/locking/lockdep.c:5049
       lock_acquire+0x20c/0x63c kernel/locking/lockdep.c:5662
       down_read+0x64/0x300 kernel/locking/rwsem.c:1520
       inode_lock_shared include/linux/fs.h:768 [inline]
       lookup_slow+0x4c/0x80 fs/namei.c:1714
       walk_component+0x270/0x364 fs/namei.c:2006
       lookup_last fs/namei.c:2463 [inline]
       path_lookupat+0x13c/0x3d0 fs/namei.c:2487
       filename_lookup+0x1c4/0x480 fs/namei.c:2516
       kern_path+0x48/0x6c fs/namei.c:2614
       is_same_device fs/btrfs/volumes.c:695 [inline]
       device_list_add+0x129c/0x1c84 fs/btrfs/volumes.c:928
       btrfs_scan_one_device+0x368/0x57c fs/btrfs/volumes.c:1461
       btrfs_mount_root+0x45c/0x7d0 fs/btrfs/super.c:1796
       legacy_get_tree+0xd4/0x16c fs/fs_context.c:632
       vfs_get_tree+0x90/0x274 fs/super.c:1573
       fc_mount fs/namespace.c:1048 [inline]
       vfs_kern_mount+0xdc/0x178 fs/namespace.c:1078
       btrfs_mount+0x2dc/0x974 fs/btrfs/super.c:1894
       legacy_get_tree+0xd4/0x16c fs/fs_context.c:632
       vfs_get_tree+0x90/0x274 fs/super.c:1573
       do_new_mount+0x228/0x810 fs/namespace.c:3078
       path_mount+0x5bc/0xe80 fs/namespace.c:3408
       do_mount fs/namespace.c:3421 [inline]
       __do_sys_mount fs/namespace.c:3629 [inline]
       __se_sys_mount fs/namespace.c:3606 [inline]
       __arm64_sys_mount+0x49c/0x59c fs/namespace.c:3606
       __invoke_syscall arch/arm64/kernel/syscall.c:38 [inline]
       invoke_syscall+0x98/0x2b4 arch/arm64/kernel/syscall.c:52
       el0_svc_common+0x138/0x258 arch/arm64/kernel/syscall.c:140
       do_el0_svc+0x58/0x130 arch/arm64/kernel/syscall.c:204
       el0_svc+0x58/0x128 arch/arm64/kernel/entry-common.c:637
       el0t_64_sync_handler+0x84/0xf0 arch/arm64/kernel/entry-common.c:655
       el0t_64_sync+0x18c/0x190 arch/arm64/kernel/entry.S:585

other info that might help us debug this:

Chain exists of:
  &type->i_mutex_dir_key#2 --> &fs_info->reloc_mutex --> &fs_devs->device_list_mutex

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&fs_devs->device_list_mutex);
                               lock(&fs_info->reloc_mutex);
                               lock(&fs_devs->device_list_mutex);
  lock(&type->i_mutex_dir_key#2);

 *** DEADLOCK ***

2 locks held by syz.5.749/8429:
 #0: ffff800015923c28 (uuid_mutex){+.+.}-{3:3}, at: btrfs_mount_root+0x1fc/0x7d0 fs/btrfs/super.c:1789
 #1: ffff0000f59464e0 (&fs_devs->device_list_mutex){+.+.}-{3:3}, at: device_list_add+0x7f4/0x1c84 fs/btrfs/volumes.c:860

stack backtrace:
CPU: 1 PID: 8429 Comm: syz.5.749 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/26/2026
Call trace:
 dump_backtrace+0x1c0/0x1ec arch/arm64/kernel/stacktrace.c:158
 show_stack+0x2c/0x3c arch/arm64/kernel/stacktrace.c:165
 __dump_stack+0x30/0x40 lib/dump_stack.c:88
 dump_stack_lvl+0xf4/0x15c lib/dump_stack.c:106
 dump_stack+0x1c/0x5c lib/dump_stack.c:113
 print_circular_bug+0x148/0x1b0 kernel/locking/lockdep.c:2048
 check_noncircular+0x264/0x2f8 kernel/locking/lockdep.c:2170
 check_prev_add kernel/locking/lockdep.c:3090 [inline]
 check_prevs_add kernel/locking/lockdep.c:3209 [inline]
 validate_chain kernel/locking/lockdep.c:3825 [inline]
 __lock_acquire+0x2880/0x6800 kernel/locking/lockdep.c:5049
 lock_acquire+0x20c/0x63c kernel/locking/lockdep.c:5662
 down_read+0x64/0x300 kernel/locking/rwsem.c:1520
 inode_lock_shared include/linux/fs.h:768 [inline]
 lookup_slow+0x4c/0x80 fs/namei.c:1714
 walk_component+0x270/0x364 fs/namei.c:2006
 lookup_last fs/namei.c:2463 [inline]
 path_lookupat+0x13c/0x3d0 fs/namei.c:2487
 filename_lookup+0x1c4/0x480 fs/namei.c:2516
 kern_path+0x48/0x6c fs/namei.c:2614
 is_same_device fs/btrfs/volumes.c:695 [inline]
 device_list_add+0x129c/0x1c84 fs/btrfs/volumes.c:928
 btrfs_scan_one_device+0x368/0x57c fs/btrfs/volumes.c:1461
 btrfs_mount_root+0x45c/0x7d0 fs/btrfs/super.c:1796
 legacy_get_tree+0xd4/0x16c fs/fs_context.c:632
 vfs_get_tree+0x90/0x274 fs/super.c:1573
 fc_mount fs/namespace.c:1048 [inline]
 vfs_kern_mount+0xdc/0x178 fs/namespace.c:1078
 btrfs_mount+0x2dc/0x974 fs/btrfs/super.c:1894
 legacy_get_tree+0xd4/0x16c fs/fs_context.c:632
 vfs_get_tree+0x90/0x274 fs/super.c:1573
 do_new_mount+0x228/0x810 fs/namespace.c:3078
 path_mount+0x5bc/0xe80 fs/namespace.c:3408
 do_mount fs/namespace.c:3421 [inline]
 __do_sys_mount fs/namespace.c:3629 [inline]
 __se_sys_mount fs/namespace.c:3606 [inline]
 __arm64_sys_mount+0x49c/0x59c fs/namespace.c:3606
 __invoke_syscall arch/arm64/kernel/syscall.c:38 [inline]
 invoke_syscall+0x98/0x2b4 arch/arm64/kernel/syscall.c:52
 el0_svc_common+0x138/0x258 arch/arm64/kernel/syscall.c:140
 do_el0_svc+0x58/0x130 arch/arm64/kernel/syscall.c:204
 el0_svc+0x58/0x128 arch/arm64/kernel/entry-common.c:637
 el0t_64_sync_handler+0x84/0xf0 arch/arm64/kernel/entry-common.c:655
 el0t_64_sync+0x18c/0x190 arch/arm64/kernel/entry.S:585
BTRFS error: device /dev/loop5 already registered with a higher generation, found 8 expect 10