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possible deadlock in lookup_slow (2)

Status: upstream: reported on 2025/05/16 17:47
Reported-by: syzbot+8dab26395833f6231fa9@syzkaller.appspotmail.com
First crash: 177d, last: 25d
▶ ▼ Similar bugs (8)
Kernel Title Rank 🛈 Repro Cause bisect Fix bisect Count Last Reported Patched Status
upstream possible deadlock in lookup_slow (3) btrfs 4 368 15d 844d 0/29 upstream: reported on 2023/07/19 13:17
linux-5.15 possible deadlock in lookup_slow 4 11 799d 971d 0/3 auto-obsoleted due to no activity on 2023/12/11 13:10
linux-4.19 possible deadlock in lookup_slow 4 22 1974d 2400d 0/1 auto-closed as invalid on 2020/10/12 22:59
linux-6.1 possible deadlock in lookup_slow 4 1 282d 282d 0/3 auto-obsoleted due to no activity on 2025/05/11 18:32
upstream possible deadlock in lookup_slow fs 4 139 2360d 2602d 0/29 auto-closed as invalid on 2019/10/25 08:42
linux-5.15 possible deadlock in lookup_slow (2) origin:lts-only 4 C inconclusive 124 65d 697d 0/3 upstream: reported C repro on 2023/12/13 22:35
linux-4.14 possible deadlock in lookup_slow 4 C 2027 979d 2270d 0/1 upstream: reported C repro on 2019/08/24 01:53
upstream possible deadlock in lookup_slow (2) fs 4 7 2129d 2173d 0/29 auto-closed as invalid on 2020/05/11 09:17

Sample crash report:
loop2: detected capacity change from 0 to 32768
======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.2.1151/9768 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:1706

but task is already holding lock:
ffff0000d47780e0 (&fs_devs->device_list_mutex){+.+.}-{3:3}, at: device_list_add+0x7cc/0x1be0 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+0x190/0x1f38 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+0xf0/0xd00 fs/btrfs/volumes.c:8064
       commit_cowonly_roots+0x16c/0x714 fs/btrfs/transaction.c:1297
       btrfs_commit_transaction+0x10bc/0x29f0 fs/btrfs/transaction.c:2427
       btrfs_rebuild_free_space_tree+0x2dc/0x6d4 fs/btrfs/free-space-tree.c:1346
       btrfs_start_pre_rw_mount+0x2c8/0x658 fs/btrfs/disk-io.c:3237
       open_ctree+0x1ae8/0x224c fs/btrfs/disk-io.c:3834
       btrfs_fill_super+0x1b4/0x2d4 fs/btrfs/super.c:1469
       btrfs_mount_root+0x730/0x780 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:1039 [inline]
       vfs_kern_mount+0xdc/0x178 fs/namespace.c:1069
       btrfs_mount+0x2a0/0x8dc 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:3069
       path_mount+0x5b4/0xe78 fs/namespace.c:3399
       do_mount fs/namespace.c:3412 [inline]
       __do_sys_mount fs/namespace.c:3620 [inline]
       __se_sys_mount fs/namespace.c:3597 [inline]
       __arm64_sys_mount+0x49c/0x584 fs/namespace.c:3597
       __invoke_syscall arch/arm64/kernel/syscall.c:38 [inline]
       invoke_syscall+0x98/0x2bc arch/arm64/kernel/syscall.c:52
       el0_svc_common+0x138/0x258 arch/arm64/kernel/syscall.c:140
       do_el0_svc+0x58/0x13c arch/arm64/kernel/syscall.c:204
       el0_svc+0x58/0x138 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

-> #5 (&fs_info->reloc_mutex){+.+.}-{3:3}:
       __mutex_lock_common+0x190/0x1f38 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+0x434/0x104c fs/btrfs/transaction.c:730
       btrfs_join_transaction+0x30/0x40 fs/btrfs/transaction.c:780
       btrfs_finish_ordered_io+0x494/0x162c fs/btrfs/inode.c:3285
       finish_ordered_fn+0x20/0x30 fs/btrfs/ordered-data.c:287
       btrfs_work_helper+0x328/0xce4 fs/btrfs/async-thread.c:280
       process_one_work+0x7f4/0x13a8 kernel/workqueue.c:2292
       worker_thread+0x8c8/0xfbc kernel/workqueue.c:2439
       kthread+0x250/0x2d8 kernel/kthread.c:376
       ret_from_fork+0x10/0x20 arch/arm64/kernel/entry.S:850

-> #4 (btrfs_trans_num_extwriters){++++}-{0:0}:
       join_transaction+0x180/0xb6c fs/btrfs/transaction.c:303
       start_transaction+0x854/0x104c fs/btrfs/transaction.c:667
       btrfs_start_transaction+0x34/0x44 fs/btrfs/transaction.c:766
       btrfs_rebuild_free_space_tree+0xb0/0x6d4 fs/btrfs/free-space-tree.c:1319
       btrfs_start_pre_rw_mount+0x2c8/0x658 fs/btrfs/disk-io.c:3237
       open_ctree+0x1ae8/0x224c fs/btrfs/disk-io.c:3834
       btrfs_fill_super+0x1b4/0x2d4 fs/btrfs/super.c:1469
       btrfs_mount_root+0x730/0x780 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:1039 [inline]
       vfs_kern_mount+0xdc/0x178 fs/namespace.c:1069
       btrfs_mount+0x2a0/0x8dc 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:3069
       path_mount+0x5b4/0xe78 fs/namespace.c:3399
       do_mount fs/namespace.c:3412 [inline]
       __do_sys_mount fs/namespace.c:3620 [inline]
       __se_sys_mount fs/namespace.c:3597 [inline]
       __arm64_sys_mount+0x49c/0x584 fs/namespace.c:3597
       __invoke_syscall arch/arm64/kernel/syscall.c:38 [inline]
       invoke_syscall+0x98/0x2bc arch/arm64/kernel/syscall.c:52
       el0_svc_common+0x138/0x258 arch/arm64/kernel/syscall.c:140
       do_el0_svc+0x58/0x13c arch/arm64/kernel/syscall.c:204
       el0_svc+0x58/0x138 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_num_writers){++++}-{0:0}:
       __lock_release kernel/locking/lockdep.c:5376 [inline]
       lock_release+0x308/0x910 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+0x54/0x360 include/linux/fs.h:1942
       __btrfs_end_transaction+0x1f4/0x5fc fs/btrfs/transaction.c:1017
       btrfs_end_transaction+0x24/0x34 fs/btrfs/transaction.c:1051
       btrfs_dirty_inode+0x16c/0x1c0 fs/btrfs/inode.c:6291
       btrfs_update_time+0x170/0x18c fs/btrfs/inode.c:6319
       inode_update_time fs/inode.c:1958 [inline]
       __file_update_time+0xc0/0x220 fs/inode.c:2147
       file_update_time+0x26c/0x2e0 fs/inode.c:2178
       btrfs_page_mkwrite+0x394/0xb78 fs/btrfs/inode.c:8602
       do_page_mkwrite+0x13c/0x358 mm/memory.c:3011
       wp_page_shared+0x148/0x550 mm/memory.c:3360
       do_wp_page+0xca0/0xef0 mm/memory.c:3510
       handle_pte_fault mm/memory.c:5049 [inline]
       __handle_mm_fault mm/memory.c:5173 [inline]
       handle_mm_fault+0x133c/0x2fd4 mm/memory.c:5294
       __do_page_fault arch/arm64/mm/fault.c:499 [inline]
       do_page_fault+0x310/0x99c arch/arm64/mm/fault.c:583
       do_mem_abort+0x70/0x194 arch/arm64/mm/fault.c:804
       el0_da+0x70/0x154 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+0x194/0xb78 fs/btrfs/inode.c:8586
       do_page_mkwrite+0x13c/0x358 mm/memory.c:3011
       wp_page_shared+0x148/0x550 mm/memory.c:3360
       do_wp_page+0xca0/0xef0 mm/memory.c:3510
       handle_pte_fault mm/memory.c:5049 [inline]
       __handle_mm_fault mm/memory.c:5173 [inline]
       handle_mm_fault+0x133c/0x2fd4 mm/memory.c:5294
       __do_page_fault arch/arm64/mm/fault.c:499 [inline]
       do_page_fault+0x310/0x99c arch/arm64/mm/fault.c:583
       do_mem_abort+0x70/0x194 arch/arm64/mm/fault.c:804
       el0_da+0x70/0x154 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:5852
       filldir64+0x2cc/0x8fc fs/readdir.c:335
       dir_emit_dot include/linux/fs.h:3701 [inline]
       dir_emit_dots include/linux/fs.h:3712 [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/0x318 fs/readdir.c:354
       __invoke_syscall arch/arm64/kernel/syscall.c:38 [inline]
       invoke_syscall+0x98/0x2bc arch/arm64/kernel/syscall.c:52
       el0_svc_common+0x138/0x258 arch/arm64/kernel/syscall.c:140
       do_el0_svc+0x58/0x13c arch/arm64/kernel/syscall.c:204
       el0_svc+0x58/0x138 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+0x293c/0x6544 kernel/locking/lockdep.c:5049
       lock_acquire+0x20c/0x644 kernel/locking/lockdep.c:5662
       down_read+0x64/0x304 kernel/locking/rwsem.c:1520
       inode_lock_shared include/linux/fs.h:768 [inline]
       lookup_slow+0x4c/0x80 fs/namei.c:1706
       walk_component+0x270/0x364 fs/namei.c:1998
       lookup_last fs/namei.c:2455 [inline]
       path_lookupat+0x13c/0x3d0 fs/namei.c:2479
       filename_lookup+0x190/0x42c fs/namei.c:2508
       kern_path+0x48/0x6c fs/namei.c:2606
       is_same_device fs/btrfs/volumes.c:695 [inline]
       device_list_add+0x1208/0x1be0 fs/btrfs/volumes.c:928
       btrfs_scan_one_device+0x364/0x570 fs/btrfs/volumes.c:1461
       btrfs_mount_root+0x404/0x780 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:1039 [inline]
       vfs_kern_mount+0xdc/0x178 fs/namespace.c:1069
       btrfs_mount+0x2a0/0x8dc 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:3069
       path_mount+0x5b4/0xe78 fs/namespace.c:3399
       do_mount fs/namespace.c:3412 [inline]
       __do_sys_mount fs/namespace.c:3620 [inline]
       __se_sys_mount fs/namespace.c:3597 [inline]
       __arm64_sys_mount+0x49c/0x584 fs/namespace.c:3597
       __invoke_syscall arch/arm64/kernel/syscall.c:38 [inline]
       invoke_syscall+0x98/0x2bc arch/arm64/kernel/syscall.c:52
       el0_svc_common+0x138/0x258 arch/arm64/kernel/syscall.c:140
       do_el0_svc+0x58/0x13c arch/arm64/kernel/syscall.c:204
       el0_svc+0x58/0x138 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.2.1151/9768:
 #0: ffff8000157d2fa8 (uuid_mutex){+.+.}-{3:3}, at: btrfs_mount_root+0x1f4/0x780 fs/btrfs/super.c:1789
 #1: ffff0000d47780e0 (&fs_devs->device_list_mutex){+.+.}-{3:3}, at: device_list_add+0x7cc/0x1be0 fs/btrfs/volumes.c:860

stack backtrace:
CPU: 0 PID: 9768 Comm: syz.2.1151 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 06/30/2025
Call trace:
 dump_backtrace+0x1c8/0x1f4 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+0xf8/0x160 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+0x240/0x2d4 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+0x293c/0x6544 kernel/locking/lockdep.c:5049
 lock_acquire+0x20c/0x644 kernel/locking/lockdep.c:5662
 down_read+0x64/0x304 kernel/locking/rwsem.c:1520
 inode_lock_shared include/linux/fs.h:768 [inline]
 lookup_slow+0x4c/0x80 fs/namei.c:1706
 walk_component+0x270/0x364 fs/namei.c:1998
 lookup_last fs/namei.c:2455 [inline]
 path_lookupat+0x13c/0x3d0 fs/namei.c:2479
 filename_lookup+0x190/0x42c fs/namei.c:2508
 kern_path+0x48/0x6c fs/namei.c:2606
 is_same_device fs/btrfs/volumes.c:695 [inline]
 device_list_add+0x1208/0x1be0 fs/btrfs/volumes.c:928
 btrfs_scan_one_device+0x364/0x570 fs/btrfs/volumes.c:1461
 btrfs_mount_root+0x404/0x780 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:1039 [inline]
 vfs_kern_mount+0xdc/0x178 fs/namespace.c:1069
 btrfs_mount+0x2a0/0x8dc 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:3069
 path_mount+0x5b4/0xe78 fs/namespace.c:3399
 do_mount fs/namespace.c:3412 [inline]
 __do_sys_mount fs/namespace.c:3620 [inline]
 __se_sys_mount fs/namespace.c:3597 [inline]
 __arm64_sys_mount+0x49c/0x584 fs/namespace.c:3597
 __invoke_syscall arch/arm64/kernel/syscall.c:38 [inline]
 invoke_syscall+0x98/0x2bc arch/arm64/kernel/syscall.c:52
 el0_svc_common+0x138/0x258 arch/arm64/kernel/syscall.c:140
 do_el0_svc+0x58/0x13c arch/arm64/kernel/syscall.c:204
 el0_svc+0x58/0x138 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 info (device loop2): first mount of filesystem c9fe44da-de57-406a-8241-57ec7d4412cf
BTRFS info (device loop2): using crc32c (crc32c-generic) checksum algorithm
BTRFS info (device loop2): turning off barriers
BTRFS info (device loop2): setting nodatasum
BTRFS info (device loop2): setting incompat feature flag for COMPRESS_ZSTD (0x10)
BTRFS info (device loop2): use zstd compression, level 3
BTRFS info (device loop2): using free space tree

Crashes (6):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2025/10/16 00:06 linux-6.1.y c2fda4b3f577 19568248 .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-6-1-kasan-arm64 possible deadlock in lookup_slow
2025/09/17 02:22 linux-6.1.y 3db754f56897 e2beed91 .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-6-1-kasan-arm64 possible deadlock in lookup_slow
2025/09/05 08:21 linux-6.1.y 28c695c365e1 d291dd2d .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-6-1-kasan-arm64 possible deadlock in lookup_slow
2025/08/11 04:55 linux-6.1.y 3594f306da12 32a0e5ed .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-6-1-kasan-arm64 possible deadlock in lookup_slow
2025/06/15 07:16 linux-6.1.y 58485ff1a74f 5f4b362d .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-6-1-kasan-arm64 possible deadlock in lookup_slow
2025/05/16 17:47 linux-6.1.y 02b72ccb5f9d cfde8269 .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-6-1-kasan-arm64 possible deadlock in lookup_slow
* Struck through repros no longer work on HEAD.