[v6.1] possible deadlock in btrfs_join_transaction (3)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: cd9b81672742 Linux 6.1.161
git tree: linux-6.1.y
console output: https://syzkaller.appspot.com/x/log.txt?x=17cc9852580000
kernel config: https://syzkaller.appspot.com/x/.config?x=b1adc0bfde2d8a4a
dashboard link: https://syzkaller.appspot.com/bug?extid=8d6efc2849cc988f03e6
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
userspace arch: arm64

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/d21df4411014/disk-cd9b8167.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/246aed0358fb/vmlinux-cd9b8167.xz
kernel image: https://storage.googleapis.com/syzbot-assets/a6f546504ec2/Image-cd9b8167.gz.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+8d6efc...@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.3.947/7803 is trying to acquire lock:
ffff0000dab6e650 (sb_internal#2){.+.+}-{0:0}, at: btrfs_join_transaction+0x30/0x40 fs/btrfs/transaction.c:780

but task is already holding lock:
ffff0000d98f4348 (&mm->mmap_lock){++++}-{3:3}, at: mmap_write_lock_killable include/linux/mmap_lock.h:87 [inline]
ffff0000d98f4348 (&mm->mmap_lock){++++}-{3:3}, at: vm_mmap_pgoff+0x144/0x294 mm/util.c:518

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #5 (&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

-> #4 (&type->i_mutex_dir_key#2){++++}-{3:3}:
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

-> #3 (&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
insert_dev_extents fs/btrfs/block-group.c:2520 [inline]
btrfs_create_pending_block_groups+0x470/0xda0 fs/btrfs/block-group.c:2571
__btrfs_end_transaction+0x134/0x5f0 fs/btrfs/transaction.c:1012
btrfs_end_transaction+0x24/0x34 fs/btrfs/transaction.c:1051
find_free_extent_update_loop fs/btrfs/extent-tree.c:4151 [inline]
find_free_extent+0x39ec/0x471c fs/btrfs/extent-tree.c:4528
btrfs_reserve_extent+0x244/0x520 fs/btrfs/extent-tree.c:4621
cow_file_range+0x3e8/0xbb8 fs/btrfs/inode.c:1276
fallback_to_cow+0x490/0xa08 fs/btrfs/inode.c:1756
run_delalloc_nocow+0xb50/0xce8 fs/btrfs/inode.c:2163
btrfs_run_delalloc_range+0x33c/0xf0c fs/btrfs/inode.c:2225
writepage_delalloc+0x11c/0x2f4 fs/btrfs/extent_io.c:1980
__extent_writepage+0x528/0xc18 fs/btrfs/extent_io.c:2284
extent_write_cache_pages fs/btrfs/extent_io.c:3199 [inline]
extent_writepages+0x93c/0x1074 fs/btrfs/extent_io.c:3321
btrfs_writepages+0x28/0x38 fs/btrfs/inode.c:8303
do_writepages+0x2b0/0x504 mm/page-writeback.c:2491
filemap_fdatawrite_wbc+0x124/0x174 mm/filemap.c:388
start_delalloc_inodes+0x598/0x91c fs/btrfs/inode.c:9827
btrfs_start_delalloc_roots+0x528/0x758 fs/btrfs/inode.c:9906
shrink_delalloc fs/btrfs/space-info.c:622 [inline]
flush_space+0x4b8/0xc94 fs/btrfs/space-info.c:730
btrfs_async_reclaim_metadata_space+0x25c/0x308 fs/btrfs/space-info.c:1080
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 (btrfs_trans_num_extwriters){++++}-{0:0}:
join_transaction+0x180/0xb68 fs/btrfs/transaction.c:303
start_transaction+0x854/0x1048 fs/btrfs/transaction.c:667
btrfs_start_transaction+0x34/0x44 fs/btrfs/transaction.c:766
btrfs_rebuild_free_space_tree+0xbc/0x6d8 fs/btrfs/free-space-tree.c:1320
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

-> #1 (btrfs_trans_num_writers){++++}-{0:0}:
join_transaction+0x154/0xb68 fs/btrfs/transaction.c:302
start_transaction+0x854/0x1048 fs/btrfs/transaction.c:667
btrfs_start_transaction+0x34/0x44 fs/btrfs/transaction.c:766
btrfs_rebuild_free_space_tree+0xbc/0x6d8 fs/btrfs/free-space-tree.c:1320
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

-> #0 (sb_internal#2){.+.+}-{0:0}:
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
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

other info that might help us debug this:

Chain exists of:
sb_internal#2 --> &type->i_mutex_dir_key#2 --> &mm->mmap_lock

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&mm->mmap_lock);
lock(&type->i_mutex_dir_key#2);
lock(&mm->mmap_lock);
lock(sb_internal#2);

*** DEADLOCK ***

2 locks held by syz.3.947/7803:
#0: ffff0000d98f4348 (&mm->mmap_lock){++++}-{3:3}, at: mmap_write_lock_killable include/linux/mmap_lock.h:87 [inline]
#0: ffff0000d98f4348 (&mm->mmap_lock){++++}-{3:3}, at: vm_mmap_pgoff+0x144/0x294 mm/util.c:518
#1: ffff0000dab6e460 (sb_writers#22){.+.+}-{0:0}, at: file_accessed include/linux/fs.h:2600 [inline]
#1: ffff0000dab6e460 (sb_writers#22){.+.+}-{0:0}, at: btrfs_file_mmap+0xb0/0x11c fs/btrfs/file.c:2053

stack backtrace:
CPU: 0 PID: 7803 Comm: syz.3.947 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
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


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