syzbot

BTRFS info (device loop2): turning on async discard
BTRFS info (device loop2): enabling free space tree
BTRFS info (device loop2): force clearing of disk cache
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======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.2.750/10696 is trying to acquire lock:
ffff888019c4f238 (&root->kernfs_rwsem){++++}-{4:4}, at: kernfs_add_one+0x41/0x5c0 fs/kernfs/dir.c:813

but task is already holding lock:
ffff88806399a2e8 (&fs_info->qgroup_ioctl_lock){+.+.}-{4:4}, at: btrfs_quota_enable+0x237/0x2850 fs/btrfs/qgroup.c:1030

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #6 (&fs_info->qgroup_ioctl_lock){+.+.}-{4:4}:
       __mutex_lock_common kernel/locking/rtmutex_api.c:533 [inline]
       mutex_lock_nested+0x5a/0x1d0 kernel/locking/rtmutex_api.c:552
       btrfs_create_qgroup+0x55/0x520 fs/btrfs/qgroup.c:1684
       create_pending_snapshot+0x948/0x3af0 fs/btrfs/transaction.c:1750
       create_pending_snapshots+0x17c/0x1c0 fs/btrfs/transaction.c:1958
       btrfs_commit_transaction+0xf75/0x31a0 fs/btrfs/transaction.c:2455
       create_snapshot fs/btrfs/ioctl.c:785 [inline]
       btrfs_mksubvol+0x9bc/0x10a0 fs/btrfs/ioctl.c:863
       btrfs_mksnapshot+0xab/0xf0 fs/btrfs/ioctl.c:905
       __btrfs_ioctl_snap_create+0x534/0x750 fs/btrfs/ioctl.c:1169
       btrfs_ioctl_snap_create+0x134/0x180 fs/btrfs/ioctl.c:1195
       btrfs_ioctl+0x44a/0xd00 fs/btrfs/ioctl.c:-1
       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+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #5 (&fs_info->reloc_mutex){+.+.}-{4:4}:
       __mutex_lock_common kernel/locking/rtmutex_api.c:533 [inline]
       mutex_lock_nested+0x5a/0x1d0 kernel/locking/rtmutex_api.c:552
       btrfs_commit_transaction+0xf68/0x31a0 fs/btrfs/transaction.c:2448
       btrfs_rebuild_free_space_tree+0x291/0x6e0 fs/btrfs/free-space-tree.c:1380
       btrfs_start_pre_rw_mount+0x1529/0x1f20 fs/btrfs/disk-io.c:3071
       open_ctree+0x2e07/0x4220 fs/btrfs/disk-io.c:3668
       btrfs_fill_super fs/btrfs/super.c:981 [inline]
       btrfs_get_tree_super fs/btrfs/super.c:1944 [inline]
       btrfs_get_tree_subvol fs/btrfs/super.c:2087 [inline]
       btrfs_get_tree+0x1061/0x1930 fs/btrfs/super.c:2121
       vfs_get_tree+0x92/0x2a0 fs/super.c:1754
       fc_mount fs/namespace.c:1193 [inline]
       do_new_mount_fc fs/namespace.c:3763 [inline]
       do_new_mount+0x341/0xd30 fs/namespace.c:3839
       do_mount fs/namespace.c:4172 [inline]
       __do_sys_mount fs/namespace.c:4361 [inline]
       __se_sys_mount+0x31d/0x420 fs/namespace.c:4338
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #4 (btrfs_trans_unblocked){++++}-{0:0}:
       wait_current_trans+0x269/0x590 fs/btrfs/transaction.c:534
       start_transaction+0x6d7/0x1650 fs/btrfs/transaction.c:710
       btrfs_dirty_inode+0x9f/0x190 fs/btrfs/inode.c:6443
       touch_atime+0x2f8/0x6b0 fs/inode.c:2271
       file_accessed include/linux/fs.h:2263 [inline]
       iterate_dir+0x44a/0x580 fs/readdir.c:111
       __do_sys_getdents fs/readdir.c:327 [inline]
       __se_sys_getdents+0xf1/0x270 fs/readdir.c:312
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #3 (sb_internal#2){.+.+}-{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/super.h:19 [inline]
       sb_start_intwrite include/linux/fs/super.h:177 [inline]
       start_transaction+0x56e/0x1650 fs/btrfs/transaction.c:702
       btrfs_dirty_inode+0x9f/0x190 fs/btrfs/inode.c:6443
       file_update_time_flags+0x2d5/0x4b0 fs/inode.c:2426
       btrfs_page_mkwrite+0x5df/0x1b10 fs/btrfs/file.c:1914
       do_page_mkwrite+0x150/0x310 mm/memory.c:3589
       wp_page_shared mm/memory.c:3990 [inline]
       do_wp_page+0x203b/0x49d0 mm/memory.c:4209
       handle_pte_fault mm/memory.c:6333 [inline]
       __handle_mm_fault mm/memory.c:6455 [inline]
       handle_mm_fault+0x9f5/0x13c0 mm/memory.c:6624
       do_user_addr_fault+0x75b/0x1340 arch/x86/mm/fault.c:1385
       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
       stac arch/x86/include/asm/uaccess_64.h:67 [inline]
       epoll_put_uevent include/linux/eventpoll.h:85 [inline]
       ep_send_events fs/eventpoll.c:1826 [inline]
       ep_try_send_events+0x79d/0xe80 fs/eventpoll.c:1909
       ep_poll fs/eventpoll.c:1974 [inline]
       do_epoll_wait+0x58f/0xbd0 fs/eventpoll.c:2466
       do_epoll_pwait fs/eventpoll.c:2496 [inline]
       __do_sys_epoll_pwait fs/eventpoll.c:2509 [inline]
       __se_sys_epoll_pwait+0x1a2/0x3a0 fs/eventpoll.c:2503
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #2 (sb_pagefaults#2){.+.+}-{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/super.h:19 [inline]
       sb_start_pagefault include/linux/fs/super.h:159 [inline]
       btrfs_page_mkwrite+0x329/0x1b10 fs/btrfs/file.c:1873
       do_page_mkwrite+0x150/0x310 mm/memory.c:3589
       wp_page_shared mm/memory.c:3990 [inline]
       do_wp_page+0x203b/0x49d0 mm/memory.c:4209
       handle_pte_fault mm/memory.c:6333 [inline]
       __handle_mm_fault mm/memory.c:6455 [inline]
       handle_mm_fault+0x9f5/0x13c0 mm/memory.c:6624
       do_user_addr_fault+0x75b/0x1340 arch/x86/mm/fault.c:1385
       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
       stac arch/x86/include/asm/uaccess_64.h:67 [inline]
       epoll_put_uevent include/linux/eventpoll.h:85 [inline]
       ep_send_events fs/eventpoll.c:1826 [inline]
       ep_try_send_events+0x79d/0xe80 fs/eventpoll.c:1909
       ep_poll fs/eventpoll.c:1974 [inline]
       do_epoll_wait+0x58f/0xbd0 fs/eventpoll.c:2466
       do_epoll_pwait fs/eventpoll.c:2496 [inline]
       __do_sys_epoll_pwait fs/eventpoll.c:2509 [inline]
       __se_sys_epoll_pwait+0x1a2/0x3a0 fs/eventpoll.c:2503
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #1 (&mm->mmap_lock){++++}-{4:4}:
       down_read_killable+0xa4/0x220 kernel/locking/rwsem.c:1560
       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
       filldir+0x2b5/0x690 fs/readdir.c:295
       dir_emit include/linux/fs.h:3566 [inline]
       kernfs_fop_readdir+0x578/0x950 fs/kernfs/dir.c:1943
       iterate_dir+0x3a5/0x580 fs/readdir.c:108
       __do_sys_getdents fs/readdir.c:327 [inline]
       __se_sys_getdents+0xf1/0x270 fs/readdir.c:312
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (&root->kernfs_rwsem){++++}-{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+0xf0/0x2e0 kernel/locking/lockdep.c:5868
       down_write+0x3a/0x50 kernel/locking/rwsem.c:1590
       kernfs_add_one+0x41/0x5c0 fs/kernfs/dir.c:813
       kernfs_create_dir_ns+0xde/0x130 fs/kernfs/dir.c:1118
       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:3540
       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+0x14d/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:
  &root->kernfs_rwsem --> &fs_info->reloc_mutex --> &fs_info->qgroup_ioctl_lock

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&fs_info->qgroup_ioctl_lock);
                               lock(&fs_info->reloc_mutex);
                               lock(&fs_info->qgroup_ioctl_lock);
  lock(&root->kernfs_rwsem);

 *** DEADLOCK ***

3 locks held by syz.2.750/10696:
 #0: ffff88803cbb6480 (sb_writers#12){.+.+}-{0:0}, at: mnt_want_write_file+0x63/0x210 fs/namespace.c:537
 #1: ffff8880639991b8 (&fs_info->subvol_sem){++++}-{4:4}, at: btrfs_ioctl_quota_ctl+0x17b/0x1c0 fs/btrfs/ioctl.c:3539
 #2: ffff88806399a2e8 (&fs_info->qgroup_ioctl_lock){+.+.}-{4:4}, at: btrfs_quota_enable+0x237/0x2850 fs/btrfs/qgroup.c:1030

stack backtrace:
CPU: 1 UID: 0 PID: 10696 Comm: syz.2.750 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+0xf0/0x2e0 kernel/locking/lockdep.c:5868
 down_write+0x3a/0x50 kernel/locking/rwsem.c:1590
 kernfs_add_one+0x41/0x5c0 fs/kernfs/dir.c:813
 kernfs_create_dir_ns+0xde/0x130 fs/kernfs/dir.c:1118
 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:3540
 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+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fb61b9ac819
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:00007fb619c06028 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007fb61bc25fa0 RCX: 00007fb61b9ac819
RDX: 00002000000000c0 RSI: 00000000c0109428 RDI: 0000000000000003
RBP: 00007fb61ba42c91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fb61bc26038 R14: 00007fb61bc25fa0 R15: 00007ffc34b143c8
 </TASK>
BTRFS info (device loop2): setting incompat feature flag for SIMPLE_QUOTA (0x10000)