syzbot

BTRFS info (device loop8): turning on async discard
BTRFS info (device loop8): enabling free space tree
BTRFS info (device loop8): force clearing of disk cache
======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.8.953/12232 is trying to acquire lock:
ffff8880243b22e8 (&fs_info->qgroup_ioctl_lock){+.+.}-{4:4}, at: btrfs_quota_enable+0x2c6/0x2850 fs/btrfs/qgroup.c:1063

but task is already holding lock:
ffff8880243b3138 (btrfs_trans_num_extwriters){++++}-{0:0}, at: join_transaction+0x154/0xd50 fs/btrfs/transaction.c:323

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+0x192/0xd50 fs/btrfs/transaction.c:324
       start_transaction+0xbe9/0x1820 fs/btrfs/transaction.c:719
       btrfs_uuid_scan_kthread+0x5df/0xb00 fs/btrfs/uuid-tree.c:456
       kthread+0x388/0x470 kernel/kthread.c:436
       ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #3 (btrfs_trans_num_writers){++++}-{0:0}:
       __lock_release kernel/locking/lockdep.c:5574 [inline]
       lock_release+0x199/0x3c0 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+0x248/0x650 fs/btrfs/transaction.c:1096
       btrfs_dirty_inode+0x14c/0x190 fs/btrfs/inode.c:6455
       touch_atime+0x2f8/0x6b0 fs/inode.c:2265
       file_accessed include/linux/fs.h:2271 [inline]
       btrfs_file_mmap_prepare+0x189/0x200 fs/btrfs/file.c:2051
       vfs_mmap_prepare include/linux/fs.h:2083 [inline]
       call_mmap_prepare mm/vma.c:2672 [inline]
       __mmap_region mm/vma.c:2755 [inline]
       mmap_region+0xec0/0x2280 mm/vma.c:2856
       do_mmap+0xc2f/0x10c0 mm/mmap.c:560
       vm_mmap_pgoff+0x2cc/0x4f0 mm/util.c:581
       ksys_mmap_pgoff+0x4e8/0x720 mm/mmap.c:606
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x15f/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #2 (&mm->mmap_lock){++++}-{4:4}:
       down_read_killable+0xa4/0x220 kernel/locking/rwsem.c:1593
       mmap_read_lock_killable+0x1d/0x70 include/linux/mmap_lock.h:601
       get_mmap_lock_carefully mm/mmap_lock.c:450 [inline]
       lock_mm_and_find_vma+0x2d7/0x340 mm/mmap_lock.c:501
       do_user_addr_fault+0x330/0x1340 arch/x86/mm/fault.c:1357
       handle_page_fault arch/x86/mm/fault.c:1474 [inline]
       exc_page_fault+0x6a/0xc0 arch/x86/mm/fault.c:1527
       asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
       filldir64+0x2be/0x640 fs/readdir.c:370
       dir_emit include/linux/fs.h:3585 [inline]
       kernfs_fop_readdir+0x578/0x950 fs/kernfs/dir.c:2000
       iterate_dir+0x3a5/0x580 fs/readdir.c:110
       __do_sys_getdents64 fs/readdir.c:399 [inline]
       __se_sys_getdents64+0xf1/0x280 fs/readdir.c:384
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x15f/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #1 (&root->kernfs_rwsem){++++}-{4:4}:
       down_write+0x3a/0x50 kernel/locking/rwsem.c:1625
       kernfs_add_one+0x41/0x5c0 fs/kernfs/dir.c:826
       kernfs_create_dir_ns+0xde/0x130 fs/kernfs/dir.c:1131
       sysfs_create_dir_ns+0x12f/0x2a0 fs/sysfs/dir.c:59
       create_dir lib/kobject.c:73 [inline]
       kobject_add_internal+0x631/0xd10 lib/kobject.c:240
       kobject_add_varg lib/kobject.c:374 [inline]
       kobject_init_and_add+0x12b/0x1a0 lib/kobject.c:457
       btrfs_sysfs_add_qgroups+0x116/0x2e0 fs/btrfs/sysfs.c:2604
       btrfs_quota_enable+0x282/0x2850 fs/btrfs/qgroup.c:1034
       btrfs_ioctl_quota_ctl+0x186/0x1c0 fs/btrfs/ioctl.c:3541
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:597 [inline]
       __se_sys_ioctl+0xff/0x170 fs/ioctl.c:583
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x15f/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+0x15a5/0x2cf0 kernel/locking/lockdep.c:5237
       lock_acquire+0x106/0x350 kernel/locking/lockdep.c:5868
       __mutex_lock_common kernel/locking/rtmutex_api.c:534 [inline]
       mutex_lock_nested+0x5a/0x1d0 kernel/locking/rtmutex_api.c:553
       btrfs_quota_enable+0x2c6/0x2850 fs/btrfs/qgroup.c:1063
       btrfs_ioctl_quota_ctl+0x186/0x1c0 fs/btrfs/ioctl.c:3541
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:597 [inline]
       __se_sys_ioctl+0xff/0x170 fs/ioctl.c:583
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x15f/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 ***

5 locks held by syz.8.953/12232:
 #0: ffff88803c076480 (sb_writers#21){.+.+}-{0:0}, at: mnt_want_write_file+0x63/0x210 fs/namespace.c:537
 #1: ffff8880243b11b8 (&fs_info->subvol_sem){+.+.}-{4:4}, at: btrfs_ioctl_quota_ctl+0x17b/0x1c0 fs/btrfs/ioctl.c:3540
 #2: ffff88803c076770 (sb_internal#2){.+.+}-{0:0}, at: btrfs_quota_enable+0x2b9/0x2850 fs/btrfs/qgroup.c:1061
 #3: ffff8880243b3110 (btrfs_trans_num_writers){++++}-{0:0}, at: join_transaction+0x154/0xd50 fs/btrfs/transaction.c:323
 #4: ffff8880243b3138 (btrfs_trans_num_extwriters){++++}-{0:0}, at: join_transaction+0x154/0xd50 fs/btrfs/transaction.c:323

stack backtrace:
CPU: 0 UID: 0 PID: 12232 Comm: syz.8.953 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 print_circular_bug+0x2e1/0x300 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+0x15a5/0x2cf0 kernel/locking/lockdep.c:5237
 lock_acquire+0x106/0x350 kernel/locking/lockdep.c:5868
 __mutex_lock_common kernel/locking/rtmutex_api.c:534 [inline]
 mutex_lock_nested+0x5a/0x1d0 kernel/locking/rtmutex_api.c:553
 btrfs_quota_enable+0x2c6/0x2850 fs/btrfs/qgroup.c:1063
 btrfs_ioctl_quota_ctl+0x186/0x1c0 fs/btrfs/ioctl.c:3541
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl+0xff/0x170 fs/ioctl.c:583
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x15f/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f582be8c819
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 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 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f582a0e6028 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f582c105fa0 RCX: 00007f582be8c819
RDX: 00002000000000c0 RSI: 00000000c0109428 RDI: 0000000000000003
RBP: 00007f582bf22c91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f582c106038 R14: 00007f582c105fa0 R15: 00007ffde7177bd8
 </TASK>
BTRFS info (device loop8): setting incompat feature flag for SIMPLE_QUOTA (0x10000)