[syzbot] [btrfs?] possible deadlock in join_transaction (3)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 05c93f3395ed Merge branch 'for-next/core' into for-kernelci
git tree: git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-kernelci
console output: https://syzkaller.appspot.com/x/log.txt?x=14386ab4580000
kernel config: https://syzkaller.appspot.com/x/.config?x=3b5338ad1e59a06c
dashboard link: https://syzkaller.appspot.com/bug?extid=c597e3e4382fe9c65845
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
userspace arch: arm64

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/6b5c913e373c/disk-05c93f33.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/15e75f1266ef/vmlinux-05c93f33.xz
kernel image: https://storage.googleapis.com/syzbot-assets/dd930129c578/Image-05c93f33.gz.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+c597e3...@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
kworker/u8:9/7952 is trying to acquire lock:
ffff0000f7ac2538 (btrfs_trans_num_writers){++++}-{0:0}, at: join_transaction+0x134/0xb5c fs/btrfs/transaction.c:317

but task is already holding lock:
ffff0000d5af8610 (sb_internal#4){.+.+}-{0:0}, at: btrfs_start_transaction+0x34/0x44 fs/btrfs/transaction.c:816

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #5 (sb_internal#4){.+.+}-{0:0}:
percpu_down_read_internal include/linux/percpu-rwsem.h:53 [inline]
percpu_down_read_freezable include/linux/percpu-rwsem.h:83 [inline]
__sb_start_write include/linux/fs.h:1916 [inline]
sb_start_intwrite include/linux/fs.h:2099 [inline]
start_transaction+0x5bc/0x155c fs/btrfs/transaction.c:699
btrfs_join_transaction+0x30/0x40 fs/btrfs/transaction.c:830
btrfs_dirty_inode+0x90/0x190 fs/btrfs/inode.c:6272
btrfs_update_time+0xa0/0xd0 fs/btrfs/inode.c:6306
inode_update_time fs/inode.c:2129 [inline]
touch_atime+0x2e4/0x818 fs/inode.c:2202
file_accessed include/linux/fs.h:2673 [inline]
btrfs_file_mmap_prepare+0xc4/0x130 fs/btrfs/file.c:2049
vfs_mmap_prepare include/linux/fs.h:2410 [inline]
call_mmap_prepare mm/vma.c:2586 [inline]
__mmap_region mm/vma.c:2654 [inline]
mmap_region+0x940/0x1d28 mm/vma.c:2740
do_mmap+0x968/0xfac mm/mmap.c:558
vm_mmap_pgoff+0x2d8/0x45c mm/util.c:581
ksys_mmap_pgoff+0x394/0x5b8 mm/mmap.c:604
__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:35 [inline]
invoke_syscall+0x98/0x254 arch/arm64/kernel/syscall.c:49
el0_svc_common+0xe8/0x23c arch/arm64/kernel/syscall.c:132
do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:151
el0_svc+0x5c/0x254 arch/arm64/kernel/entry-common.c:724
el0t_64_sync_handler+0x84/0x12c arch/arm64/kernel/entry-common.c:743
el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:596

-> #4 (&mm->mmap_lock){++++}-{4:4}:
down_read_killable+0x60/0x32c kernel/locking/rwsem.c:1560
mmap_read_lock_killable+0x28/0x8c include/linux/mmap_lock.h:377
get_mmap_lock_carefully mm/mmap_lock.c:377 [inline]
lock_mm_and_find_vma+0x2a4/0x2d8 mm/mmap_lock.c:428
do_page_fault+0x50c/0x13cc arch/arm64/mm/fault.c:678
do_translation_fault+0xc4/0x114 arch/arm64/mm/fault.c:793
do_mem_abort+0x70/0x194 arch/arm64/mm/fault.c:933
el1_abort+0x40/0x64 arch/arm64/kernel/entry-common.c:303
el1h_64_sync_handler+0x50/0xfc arch/arm64/kernel/entry-common.c:437
el1h_64_sync+0x6c/0x70 arch/arm64/kernel/entry.S:591
__uaccess_mask_ptr arch/arm64/include/asm/uaccess.h:169 [inline]
filldir64+0x2ec/0x6bc fs/readdir.c:379
dir_emit include/linux/fs.h:3988 [inline]
kernfs_fop_readdir+0x498/0x79c fs/kernfs/dir.c:1910
iterate_dir+0x2dc/0x478 fs/readdir.c:108
__do_sys_getdents64 fs/readdir.c:410 [inline]
__se_sys_getdents64 fs/readdir.c:396 [inline]
__arm64_sys_getdents64+0x110/0x2fc fs/readdir.c:396
__invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
invoke_syscall+0x98/0x254 arch/arm64/kernel/syscall.c:49
el0_svc_common+0xe8/0x23c arch/arm64/kernel/syscall.c:132
do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:151
el0_svc+0x5c/0x254 arch/arm64/kernel/entry-common.c:724
el0t_64_sync_handler+0x84/0x12c arch/arm64/kernel/entry-common.c:743
el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:596

-> #3 (&root->kernfs_rwsem){++++}-{4:4}:
down_write+0x50/0xc0 kernel/locking/rwsem.c:1590
kernfs_add_one+0x48/0x60c fs/kernfs/dir.c:791
kernfs_create_dir_ns+0xd4/0x12c fs/kernfs/dir.c:1093
sysfs_create_dir_ns+0x114/0x24c fs/sysfs/dir.c:59
create_dir lib/kobject.c:73 [inline]
kobject_add_internal+0x5a8/0xb20 lib/kobject.c:240
kobject_add_varg lib/kobject.c:374 [inline]
kobject_init_and_add+0x118/0x17c lib/kobject.c:457
btrfs_sysfs_add_qgroups+0x110/0x268 fs/btrfs/sysfs.c:2645
btrfs_quota_enable+0x210/0x2438 fs/btrfs/qgroup.c:1022
btrfs_ioctl_quota_ctl+0x178/0x1bc fs/btrfs/ioctl.c:3667
btrfs_ioctl+0x86c/0xc3c fs/btrfs/ioctl.c:5333
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl fs/ioctl.c:583 [inline]
__arm64_sys_ioctl+0x14c/0x1c4 fs/ioctl.c:583
__invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
invoke_syscall+0x98/0x254 arch/arm64/kernel/syscall.c:49
el0_svc_common+0xe8/0x23c arch/arm64/kernel/syscall.c:132
do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:151
el0_svc+0x5c/0x254 arch/arm64/kernel/entry-common.c:724
el0t_64_sync_handler+0x84/0x12c arch/arm64/kernel/entry-common.c:743
el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:596

-> #2 (&fs_info->qgroup_ioctl_lock){+.+.}-{4:4}:
__mutex_lock_common+0x1d0/0x2678 kernel/locking/mutex.c:598
__mutex_lock kernel/locking/mutex.c:760 [inline]
mutex_lock_nested+0x2c/0x38 kernel/locking/mutex.c:812
btrfs_quota_enable+0x24c/0x2438 fs/btrfs/qgroup.c:1051
btrfs_ioctl_quota_ctl+0x178/0x1bc fs/btrfs/ioctl.c:3667
btrfs_ioctl+0x86c/0xc3c fs/btrfs/ioctl.c:5333
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl fs/ioctl.c:583 [inline]
__arm64_sys_ioctl+0x14c/0x1c4 fs/ioctl.c:583
__invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
invoke_syscall+0x98/0x254 arch/arm64/kernel/syscall.c:49
el0_svc_common+0xe8/0x23c arch/arm64/kernel/syscall.c:132
do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:151
el0_svc+0x5c/0x254 arch/arm64/kernel/entry-common.c:724
el0t_64_sync_handler+0x84/0x12c arch/arm64/kernel/entry-common.c:743
el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:596

-> #1 (btrfs_trans_num_extwriters){++++}-{0:0}:
join_transaction+0x190/0xb5c fs/btrfs/transaction.c:321
start_transaction+0x778/0x155c fs/btrfs/transaction.c:705
btrfs_start_transaction+0x34/0x44 fs/btrfs/transaction.c:816
btrfs_rebuild_free_space_tree+0xac/0x6c0 fs/btrfs/free-space-tree.c:1340
btrfs_start_pre_rw_mount+0xed8/0x1728 fs/btrfs/disk-io.c:3062
open_ctree+0x24cc/0x358c fs/btrfs/disk-io.c:3619
btrfs_fill_super fs/btrfs/super.c:987 [inline]
btrfs_get_tree_super fs/btrfs/super.c:1951 [inline]
btrfs_get_tree_subvol fs/btrfs/super.c:2094 [inline]
btrfs_get_tree+0xd94/0x15dc fs/btrfs/super.c:2128
vfs_get_tree+0x90/0x28c fs/super.c:1758
fc_mount fs/namespace.c:1199 [inline]
do_new_mount_fc fs/namespace.c:3642 [inline]
do_new_mount+0x284/0x944 fs/namespace.c:3718
path_mount+0x5b4/0xdfc fs/namespace.c:4028
do_mount fs/namespace.c:4041 [inline]
__do_sys_mount fs/namespace.c:4229 [inline]
__se_sys_mount fs/namespace.c:4206 [inline]
__arm64_sys_mount+0x3e8/0x468 fs/namespace.c:4206
__invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
invoke_syscall+0x98/0x254 arch/arm64/kernel/syscall.c:49
el0_svc_common+0xe8/0x23c arch/arm64/kernel/syscall.c:132
do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:151
el0_svc+0x5c/0x254 arch/arm64/kernel/entry-common.c:724
el0t_64_sync_handler+0x84/0x12c arch/arm64/kernel/entry-common.c:743
el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:596

-> #0 (btrfs_trans_num_writers){++++}-{0:0}:
check_prev_add kernel/locking/lockdep.c:3165 [inline]
check_prevs_add kernel/locking/lockdep.c:3284 [inline]
validate_chain kernel/locking/lockdep.c:3908 [inline]
__lock_acquire+0x1774/0x30a4 kernel/locking/lockdep.c:5237
lock_acquire+0x14c/0x2e0 kernel/locking/lockdep.c:5868
join_transaction+0x164/0xb5c fs/btrfs/transaction.c:320
start_transaction+0x778/0x155c fs/btrfs/transaction.c:705
btrfs_start_transaction+0x34/0x44 fs/btrfs/transaction.c:816
btrfs_qgroup_rescan_worker+0x280/0x19f8 fs/btrfs/qgroup.c:3824
btrfs_work_helper+0x380/0xca8 fs/btrfs/async-thread.c:312
process_one_work+0x7e8/0x155c kernel/workqueue.c:3263
process_scheduled_works kernel/workqueue.c:3346 [inline]
worker_thread+0x958/0xed8 kernel/workqueue.c:3427
kthread+0x5fc/0x75c kernel/kthread.c:463
ret_from_fork+0x10/0x20 arch/arm64/kernel/entry.S:844

other info that might help us debug this:

Chain exists of:
btrfs_trans_num_writers --> &mm->mmap_lock --> sb_internal#4

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
rlock(sb_internal#4);
lock(&mm->mmap_lock);
lock(sb_internal#4);
rlock(btrfs_trans_num_writers);

*** DEADLOCK ***

3 locks held by kworker/u8:9/7952:
#0: ffff0000f51cb148 ((wq_completion)btrfs-qgroup-rescan){+.+.}-{0:0}, at: process_one_work+0x63c/0x155c kernel/workqueue.c:3237
#1: ffff8000a43c7be0 ((work_completion)(&work->normal_work)){+.+.}-{0:0}, at: process_one_work+0x6d4/0x155c kernel/workqueue.c:3237
#2: ffff0000d5af8610 (sb_internal#4){.+.+}-{0:0}, at: btrfs_start_transaction+0x34/0x44 fs/btrfs/transaction.c:816

stack backtrace:
CPU: 0 UID: 0 PID: 7952 Comm: kworker/u8:9 Not tainted syzkaller #0 PREEMPT
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/03/2025
Workqueue: btrfs-qgroup-rescan btrfs_work_helper
Call trace:
show_stack+0x2c/0x3c arch/arm64/kernel/stacktrace.c:499 (C)
__dump_stack+0x30/0x40 lib/dump_stack.c:94
dump_stack_lvl+0xd8/0x12c lib/dump_stack.c:120
dump_stack+0x1c/0x28 lib/dump_stack.c:129
print_circular_bug+0x324/0x32c kernel/locking/lockdep.c:2043
check_noncircular+0x154/0x174 kernel/locking/lockdep.c:2175
check_prev_add kernel/locking/lockdep.c:3165 [inline]
check_prevs_add kernel/locking/lockdep.c:3284 [inline]
validate_chain kernel/locking/lockdep.c:3908 [inline]
__lock_acquire+0x1774/0x30a4 kernel/locking/lockdep.c:5237
lock_acquire+0x14c/0x2e0 kernel/locking/lockdep.c:5868
join_transaction+0x164/0xb5c fs/btrfs/transaction.c:320
start_transaction+0x778/0x155c fs/btrfs/transaction.c:705
btrfs_start_transaction+0x34/0x44 fs/btrfs/transaction.c:816
btrfs_qgroup_rescan_worker+0x280/0x19f8 fs/btrfs/qgroup.c:3824
btrfs_work_helper+0x380/0xca8 fs/btrfs/async-thread.c:312
process_one_work+0x7e8/0x155c kernel/workqueue.c:3263
process_scheduled_works kernel/workqueue.c:3346 [inline]
worker_thread+0x958/0xed8 kernel/workqueue.c:3427
kthread+0x5fc/0x75c kernel/kthread.c:463
ret_from_fork+0x10/0x20 arch/arm64/kernel/entry.S:844
BTRFS info (device loop7): qgroup scan completed (inconsistency flag cleared)


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