syzbot

loop6: detected capacity change from 0 to 32768
======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.6.1024/9047 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:
ffff0000f511dce0 (&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:

-> #5 (&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:8065
       commit_cowonly_roots+0x170/0x71c fs/btrfs/transaction.c:1297
       btrfs_commit_transaction+0x1108/0x29fc fs/btrfs/transaction.c:2427
       btrfs_rebuild_free_space_tree+0x2e8/0x6d8 fs/btrfs/free-space-tree.c:1347
       btrfs_start_pre_rw_mount+0x2c8/0x658 fs/btrfs/disk-io.c:3237
       open_ctree+0x1e44/0x2604 fs/btrfs/disk-io.c:3834
       btrfs_fill_super+0x1b4/0x2d4 fs/btrfs/super.c:1469
       btrfs_mount_root+0x780/0x7d0 fs/btrfs/super.c:1834
       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

-> #4 (&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_commit_transaction+0xfb8/0x29fc fs/btrfs/transaction.c:2367
       btrfs_rebuild_free_space_tree+0x2e8/0x6d8 fs/btrfs/free-space-tree.c:1347
       btrfs_start_pre_rw_mount+0x2c8/0x658 fs/btrfs/disk-io.c:3237
       open_ctree+0x1e44/0x2604 fs/btrfs/disk-io.c:3834
       btrfs_fill_super+0x1b4/0x2d4 fs/btrfs/super.c:1469
       btrfs_mount_root+0x780/0x7d0 fs/btrfs/super.c:1834
       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

-> #3 (btrfs_trans_unblocked){++++}-{0:0}:
       wait_current_trans+0x1e8/0x444 fs/btrfs/transaction.c:516
       start_transaction+0x834/0x1048 fs/btrfs/transaction.c:664
       btrfs_start_transaction+0x34/0x44 fs/btrfs/transaction.c:766
       btrfs_qgroup_rescan_worker+0x1190/0x152c fs/btrfs/qgroup.c:3443
       btrfs_work_helper+0x324/0xcb0 fs/btrfs/async-thread.c:280
       process_one_work+0x7f8/0x13a4 kernel/workqueue.c:2292
       worker_thread+0x8c4/0xfec kernel/workqueue.c:2439
       kthread+0x250/0x2d8 kernel/kthread.c:376
       ret_from_fork+0x10/0x20 arch/arm64/kernel/entry.S:850

-> #2 (sb_internal#2){.+.+}-{0:0}:
       percpu_down_read include/linux/percpu-rwsem.h:51 [inline]
       __sb_start_write include/linux/fs.h:1891 [inline]
       sb_start_intwrite include/linux/fs.h:2013 [inline]
       start_transaction+0x5c4/0x1048 fs/btrfs/transaction.c:661
       btrfs_join_transaction+0x30/0x40 fs/btrfs/transaction.c:780
       btrfs_dirty_inode+0xb4/0x1c0 fs/btrfs/inode.c:6277
       btrfs_update_time+0x170/0x18c fs/btrfs/inode.c:6319
       inode_update_time fs/inode.c:1958 [inline]
       touch_atime+0x368/0x7dc fs/inode.c:2031
       file_accessed include/linux/fs.h:2600 [inline]
       btrfs_file_mmap+0xb0/0x11c fs/btrfs/file.c:2053
       call_mmap include/linux/fs.h:2270 [inline]
       mmap_file+0x6c/0xc8 mm/util.c:1109
       __mmap_region mm/mmap.c:2772 [inline]
       mmap_region+0xcec/0x1900 mm/mmap.c:2916
       do_mmap+0x9cc/0x1138 mm/mmap.c:1436
       vm_mmap_pgoff+0x190/0x294 mm/util.c:520
       ksys_mmap_pgoff+0x3b8/0x5a0 mm/mmap.c:1482
       __do_sys_mmap arch/arm64/kernel/sys.c:28 [inline]
       __se_sys_mmap arch/arm64/kernel/sys.c:21 [inline]
       __arm64_sys_mmap+0xf8/0x110 arch/arm64/kernel/sys.c:21
       __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

-> #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.6.1024/9047:
 #0: ffff800015913808 (uuid_mutex){+.+.}-{3:3}, at: btrfs_mount_root+0x1fc/0x7d0 fs/btrfs/super.c:1789
 #1: ffff0000f511dce0 (&fs_devs->device_list_mutex){+.+.}-{3:3}, at: device_list_add+0x7f4/0x1c84 fs/btrfs/volumes.c:860

stack backtrace:
CPU: 1 PID: 9047 Comm: syz.6.1024 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/03/2025
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/loop6 already registered with a higher generation, found 8 expect 12