[syzbot] [btrfs?] possible deadlock in btrfs_quota_disable

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: cc3aa43b44bd Add linux-next specific files for 20251219
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=11737222580000
kernel config: https://syzkaller.appspot.com/x/.config?x=da1bc82c6189c463
dashboard link: https://syzkaller.appspot.com/bug?extid=42f188c8a5480df414f9
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/30bf539e6f28/disk-cc3aa43b.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/0e2f8b08e342/vmlinux-cc3aa43b.xz
kernel image: https://storage.googleapis.com/syzbot-assets/ec7ee6ece11f/bzImage-cc3aa43b.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+42f188...@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Tainted: G L
------------------------------------------------------
syz.6.4382/25959 is trying to acquire lock:
ffff88807ae65990 (&fs_info->qgroup_ioctl_lock){+.+.}-{4:4}, at: btrfs_quota_disable+0x341/0x1000 fs/btrfs/qgroup.c:1369

but task is already holding lock:
ffff88807ae66550 (btrfs_trans_num_extwriters){++++}-{0:0}, at: join_transaction+0x164/0xd60 fs/btrfs/transaction.c:322

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #4 (btrfs_trans_num_extwriters){++++}-{0:0}:
join_transaction+0x1a4/0xd60 fs/btrfs/transaction.c:323
start_transaction+0x6ba/0x15f0 fs/btrfs/transaction.c:708
btrfs_create_common+0x146/0x2f0 fs/btrfs/inode.c:6920
lookup_open fs/namei.c:4439 [inline]
open_last_lookups fs/namei.c:4539 [inline]
path_openat+0x1387/0x3840 fs/namei.c:4783
do_filp_open+0x1fa/0x410 fs/namei.c:4813
file_open_name fs/open.c:1337 [inline]
filp_open+0x176/0x1d0 fs/open.c:1357
coredump_file fs/coredump.c:931 [inline]
do_coredump fs/coredump.c:1099 [inline]
vfs_coredump+0x1d6e/0x3e60 fs/coredump.c:1200
get_signal+0x1108/0x1340 kernel/signal.c:3019
arch_do_signal_or_restart+0x9a/0x7a0 arch/x86/kernel/signal.c:337
__exit_to_user_mode_loop kernel/entry/common.c:41 [inline]
exit_to_user_mode_loop kernel/entry/common.c:75 [inline]
__exit_to_user_mode_prepare include/linux/irq-entry-common.h:226 [inline]
irqentry_exit_to_user_mode_prepare include/linux/irq-entry-common.h:270 [inline]
irqentry_exit_to_user_mode include/linux/irq-entry-common.h:339 [inline]
irqentry_exit+0x177/0x660 kernel/entry/common.c:196
exc_page_fault+0xab/0x100 arch/x86/mm/fault.c:1535
asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618

-> #3 (btrfs_trans_num_writers){++++}-{0:0}:
__lock_release kernel/locking/lockdep.c:5574 [inline]
lock_release+0x1ab/0x3b0 kernel/locking/lockdep.c:5889
percpu_up_read include/linux/percpu-rwsem.h:112 [inline]
__sb_end_write include/linux/fs/super.h:14 [inline]
sb_end_intwrite+0x26/0x1c0 include/linux/fs/super.h:101
__btrfs_end_transaction+0x248/0x640 fs/btrfs/transaction.c:1084
btrfs_dirty_inode+0x14c/0x190 fs/btrfs/inode.c:6420
inode_update_time fs/inode.c:2167 [inline]
touch_atime+0x2f9/0x6d0 fs/inode.c:2240
file_accessed include/linux/fs.h:2255 [inline]
btrfs_file_mmap_prepare+0x176/0x1f0 fs/btrfs/file.c:2050
vfs_mmap_prepare include/linux/fs.h:2059 [inline]
call_mmap_prepare mm/vma.c:2596 [inline]
__mmap_region mm/vma.c:2692 [inline]
mmap_region+0xb2b/0x1d10 mm/vma.c:2786
do_mmap+0xc45/0x10d0 mm/mmap.c:558
vm_mmap_pgoff+0x2a6/0x4d0 mm/util.c:581
ksys_mmap_pgoff+0x51f/0x760 mm/mmap.c:604
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xfa/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #2 (&mm->mmap_lock){++++}-{4:4}:
down_read_killable+0x50/0x350 kernel/locking/rwsem.c:1560
mmap_read_lock_killable+0x1d/0x70 include/linux/mmap_lock.h:400
get_mmap_lock_carefully mm/mmap_lock.c:399 [inline]
lock_mm_and_find_vma+0x2a8/0x300 mm/mmap_lock.c:450
do_user_addr_fault+0x331/0x1380 arch/x86/mm/fault.c:1359
handle_page_fault arch/x86/mm/fault.c:1476 [inline]
exc_page_fault+0x82/0x100 arch/x86/mm/fault.c:1532
asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
filldir64+0x2c1/0x690 fs/readdir.c:-1
dir_emit include/linux/fs.h:3526 [inline]
kernfs_fop_readdir+0x537/0x870 fs/kernfs/dir.c:1913
iterate_dir+0x399/0x570 fs/readdir.c:108
__do_sys_getdents64 fs/readdir.c:410 [inline]
__se_sys_getdents64+0xe4/0x260 fs/readdir.c:396
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xfa/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #1 (&root->kernfs_rwsem){++++}-{4:4}:
down_write+0x96/0x1f0 kernel/locking/rwsem.c:1590
kernfs_add_one+0x41/0x5c0 fs/kernfs/dir.c:794
kernfs_create_dir_ns+0xde/0x130 fs/kernfs/dir.c:1096
sysfs_create_dir_ns+0x123/0x280 fs/sysfs/dir.c:59
create_dir lib/kobject.c:73 [inline]
kobject_add_internal+0x6ab/0xcc0 lib/kobject.c:240
kobject_add_varg lib/kobject.c:374 [inline]
kobject_init_and_add+0x125/0x190 lib/kobject.c:457
btrfs_sysfs_add_qgroups+0x111/0x2b0 fs/btrfs/sysfs.c:2643
btrfs_quota_enable+0x25d/0x2920 fs/btrfs/qgroup.c:1034
btrfs_ioctl_quota_ctl+0x183/0x1c0 fs/btrfs/ioctl.c:3613
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xfa/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (&fs_info->qgroup_ioctl_lock){+.+.}-{4:4}:
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+0x15a6/0x2cf0 kernel/locking/lockdep.c:5237
lock_acquire+0x117/0x340 kernel/locking/lockdep.c:5868
__mutex_lock_common kernel/locking/mutex.c:614 [inline]
__mutex_lock+0x187/0x1350 kernel/locking/mutex.c:776
btrfs_quota_disable+0x341/0x1000 fs/btrfs/qgroup.c:1369
btrfs_ioctl_quota_ctl+0x150/0x1c0 fs/btrfs/ioctl.c:3643
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xfa/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

Chain exists of:
&fs_info->qgroup_ioctl_lock --> btrfs_trans_num_writers --> btrfs_trans_num_extwriters

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
rlock(btrfs_trans_num_extwriters);
lock(btrfs_trans_num_writers);
lock(btrfs_trans_num_extwriters);
lock(&fs_info->qgroup_ioctl_lock);

*** DEADLOCK ***

6 locks held by syz.6.4382/25959:
#0: ffff8880308d2420 (sb_writers#24){.+.+}-{0:0}, at: mnt_want_write_file+0x60/0x200 fs/namespace.c:543
#1: ffff88807ae648e0 (&fs_info->cleaner_mutex){+.+.}-{4:4}, at: btrfs_ioctl_quota_ctl+0x139/0x1c0 fs/btrfs/ioctl.c:3641
#2: ffff88807ae64c60 (&fs_info->subvol_sem){+.+.}-{4:4}, at: btrfs_ioctl_quota_ctl+0x148/0x1c0 fs/btrfs/ioctl.c:3642
#3: ffff8880308d2610 (sb_internal#5){.+.+}-{0:0}, at: btrfs_quota_disable+0x32f/0x1000 fs/btrfs/qgroup.c:1367
#4: ffff88807ae66528 (btrfs_trans_num_writers){++++}-{0:0}, at: join_transaction+0x164/0xd60 fs/btrfs/transaction.c:322
#5: ffff88807ae66550 (btrfs_trans_num_extwriters){++++}-{0:0}, at: join_transaction+0x164/0xd60 fs/btrfs/transaction.c:322

stack backtrace:
CPU: 1 UID: 0 PID: 25959 Comm: syz.6.4382 Tainted: G L syzkaller #0 PREEMPT(full)
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
Call Trace:
<TASK>
dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
print_circular_bug+0x2e2/0x300 kernel/locking/lockdep.c:2043
check_noncircular+0x12e/0x150 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+0x15a6/0x2cf0 kernel/locking/lockdep.c:5237
lock_acquire+0x117/0x340 kernel/locking/lockdep.c:5868
__mutex_lock_common kernel/locking/mutex.c:614 [inline]
__mutex_lock+0x187/0x1350 kernel/locking/mutex.c:776
btrfs_quota_disable+0x341/0x1000 fs/btrfs/qgroup.c:1369
btrfs_ioctl_quota_ctl+0x150/0x1c0 fs/btrfs/ioctl.c:3643
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xfa/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7feaa538f749
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007feaa62dd038 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007feaa55e6090 RCX: 00007feaa538f749
RDX: 0000200000000080 RSI: 00000000c0109428 RDI: 0000000000000003
RBP: 00007feaa5413f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007feaa55e6128 R14: 00007feaa55e6090 R15: 00007ffc3f417778
</TASK>


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