[syzbot] [btrfs?] WARNING in btrfs_put_root

[syzbot <syzbot+55d1814e49f1fc0ba542@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    fa4ad798f6f6 Merge tag 'for-7.4-rc1-tag' of git://git.kern..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=16f5af35580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=d44b06612b68f56
dashboard link: https://syzkaller.appspot.com/bug?extid=55d1814e49f1fc0ba542
compiler:       Debian clang version 22.1.8 (1~deb13u4), Debian LLD 22.1.8

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/7a262c93f0a1/disk-fa4ad798.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/670f0137c169/vmlinux-fa4ad798.xz
kernel image: https://storage.googleapis.com/syzbot-assets/7b5f342645af/bzImage-fa4ad798.xz

AI-generated root cause summary:
When a Btrfs filesystem encounters a transaction abort (reported in state EA),
inodes being evicted during unmount (particularly via delayed iputs in
close_ctree(), which runs after btrfs_error_commit_super()) fail to persist
their delayed inode updates in btrfs_commit_inode_delayed_inode() because
transaction joining fails. In btrfs_evict_inode(), this error causes execution
to jump directly to the out label, bypassing btrfs_kill_delayed_inode_items().
Because the delayed items remain uncleaned, __btrfs_release_delayed_node()
re-queues the delayed node into delayed_root->node_list and retains a reference,
preventing refcount_dec_and_test() from hitting zero and skipping
xa_erase(&root->delayed_nodes, ...). When the root is finally freed in
btrfs_put_root(), root->delayed_nodes is still non-empty, triggering
WARN_ON(!xa_empty(&root->delayed_nodes)).

full analysis:  https://syzkaller.appspot.com/x/ai_root_cause?x=55d1814e49f1fc0ba542

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+55d1814e49f1fc0ba542@syzkaller.appspotmail.com

BTRFS info (device loop6 state EA): last unmount of filesystem 395ef67a-297e-477c-816d-cd80a5b93e5d
------------[ cut here ]------------
!xa_empty(&root->delayed_nodes)
WARNING: fs/btrfs/disk-io.c:1844 at btrfs_put_root+0x1f9/0x250 fs/btrfs/disk-io.c:1844, CPU#1: syz-executor/6160
Modules linked in:
CPU: 1 UID: 0 PID: 6160 Comm: syz-executor Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/06/2026
RIP: 0010:btrfs_put_root+0x1f9/0x250 fs/btrfs/disk-io.c:1844
Code: c5 0e fe e8 b9 81 a3 fd 48 8d bb 40 05 00 00 90 0f 0b 90 e8 69 98 de 07 e9 9b fe ff ff e8 9f 81 a3 fd 48 8d bb 90 05 00 00 90 <0f> 0b 90 e8 4f 98 de 07 e9 ad fe ff ff e8 85 81 a3 fd 90 0f 0b 90
RSP: 0018:ffffc90006287d18 EFLAGS: 00010293
RAX: ffffffff84258d21 RBX: ffff888079a62000 RCX: ffff8880233e8000
RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffff888079a62590
RBP: 0000000000000001 R08: ffff888079a6262b R09: 1ffff1100f34c4c5
R10: dffffc0000000000 R11: ffffed100f34c4c6 R12: dffffc0000000000
R13: ffff888069f06218 R14: ffff888079a625d8 R15: dffffc0000000000
FS:  000055556295c500(0000) GS:ffff888123509000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007faacae5e000 CR3: 000000001ef0b000 CR4: 0000000000350ef0
Call Trace:
 <TASK>
 btrfs_free_fs_info+0x252/0x360 fs/btrfs/disk-io.c:1257
 deactivate_locked_super+0xbe/0x110 fs/super.c:558
 cleanup_mnt+0x204/0x2a0 fs/namespace.c:1396
 task_work_run+0x1d9/0x260 kernel/task_work.c:233
 resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
 __exit_to_user_mode_loop kernel/entry/common.c:70 [inline]
 exit_to_user_mode_loop+0x204/0x770 kernel/entry/common.c:101
 __exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline]
 syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:230 [inline]
 syscall_exit_to_user_mode include/linux/entry-common.h:343 [inline]
 do_syscall_64+0x328/0x520 arch/x86/entry/syscall_64.c:89
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fb8687a0d57
Code: a2 c7 05 9c a9 25 00 00 00 00 00 eb 96 e8 e1 12 00 00 90 31 f6 e9 09 00 00 00 66 0f 1f 84 00 00 00 00 00 b8 a6 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 01 c3 48 c7 c2 e8 ff ff ff f7 d8 64 89 02 b8
RSP: 002b:00007ffc6dae6688 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 00007fb8687a0d57
RDX: 0000000000000000 RSI: 0000000000000009 RDI: 00007ffc6dae6740
RBP: 00007ffc6dae6740 R08: 00007ffc6dae7740 R09: 00000000ffffffff
R10: 0000000000000000 R11: 0000000000000246 R12: 00007ffc6dae77d0
R13: 00007fb868836fb3 R14: 0000000000041ae3 R15: 00007ffc6dae7810
 </TASK>


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