Freezing with imperfect legacy cgroup freezer. See cgroup.freeze of cgroup v2
loop0: detected capacity change from 0 to 32768
btrfs: Deprecated parameter 'usebackuproot'
BTRFS warning: 'usebackuproot' is deprecated, use 'rescue=usebackuproot' instead
btrfs: Deprecated parameter 'usebackuproot'
BTRFS warning: 'usebackuproot' is deprecated, use 'rescue=usebackuproot' instead
BTRFS: device fsid c9fe44da-de57-406a-8241-57ec7d4412cf devid 1 transid 8 /dev/loop0 (7:0) scanned by syz.0.0 (5346)
BTRFS info (device loop0): first mount of filesystem c9fe44da-de57-406a-8241-57ec7d4412cf
BTRFS info (device loop0): using crc32c (crc32c-lib) checksum algorithm
BTRFS info (device loop0): rebuilding free space tree
BTRFS info (device loop0): turning off barriers
BTRFS info (device loop0): turning on sync discard
BTRFS info (device loop0): enabling free space tree
BTRFS info (device loop0): force clearing of disk cache
BTRFS info (device loop0): enabling auto defrag
BTRFS info (device loop0): trying to use backup root at mount time
BTRFS error (device loop0): failed to run delayed ref for logical 8323072 num_bytes 4096 type 176 action 1 ref_mod 1: -28
------------[ cut here ]------------
BTRFS: Transaction aborted (error -28)
WARNING: fs/btrfs/extent-tree.c:2162 at btrfs_run_delayed_refs+0x30b/0x3a0 fs/btrfs/extent-tree.c:2162, CPU#0: syz.0.0/5346
Modules linked in:
CPU: 0 UID: 0 PID: 5346 Comm: syz.0.0 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
RIP: 0010:btrfs_run_delayed_refs+0x30e/0x3a0 fs/btrfs/extent-tree.c:2162
Code: 85 e4 75 07 e8 33 4f f5 fd eb 7a e8 9c 28 db fd 84 c0 74 20 e8 23 4f f5 fd eb 6a e8 1c 4f f5 fd 48 8d 3d 75 94 de 0b 44 89 fe <67> 48 0f b9 3a e9 0b ff ff ff e8 93 46 91 07 41 89 c4 31 ff 89 c6
RSP: 0018:ffffc9000db3f158 EFLAGS: 00010246
RAX: ffffffff83cc71b4 RBX: ffff8880477a76f0 RCX: 0000000000100000
RDX: ffffc90020a32000 RSI: 00000000ffffffe4 RDI: ffffffff8fab0630
RBP: 0000000000000000 R08: ffff888000b5c980 R09: 0000000000000003
R10: 00000000fffffffb R11: 0000000000000002 R12: ffff888011744001
R13: ffff888011d19000 R14: dffffc0000000000 R15: 00000000ffffffe4
FS:  00007f13b32d76c0(0000) GS:ffff88808d22a000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ff51eabbe18 CR3: 0000000035e1f000 CR4: 0000000000352ef0
Call Trace:
 <TASK>
 btrfs_commit_transaction+0x28b/0x3b10 fs/btrfs/transaction.c:2229
 btrfs_sync_file+0xd20/0x1140 fs/btrfs/file.c:1819
 generic_write_sync include/linux/fs.h:2616 [inline]
 btrfs_do_write_iter+0x689/0x820 fs/btrfs/file.c:1470
 iter_file_splice_write+0x972/0x10b0 fs/splice.c:738
 do_splice_from fs/splice.c:938 [inline]
 direct_splice_actor+0x101/0x160 fs/splice.c:1161
 splice_direct_to_actor+0x5a8/0xcc0 fs/splice.c:1105
 do_splice_direct_actor fs/splice.c:1204 [inline]
 do_splice_direct+0x181/0x270 fs/splice.c:1230
 do_sendfile+0x4da/0x7e0 fs/read_write.c:1370
 __do_sys_sendfile64 fs/read_write.c:1431 [inline]
 __se_sys_sendfile64+0x13e/0x190 fs/read_write.c:1417
 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: 0033:0x7f13b238f7c9
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:00007f13b32d7038 EFLAGS: 00000246 ORIG_RAX: 0000000000000028
RAX: ffffffffffffffda RBX: 00007f13b25e5fa0 RCX: 00007f13b238f7c9
RDX: 0000000000000000 RSI: 0000000000000007 RDI: 0000000000000007
RBP: 00007f13b2413f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000800000009 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f13b25e6038 R14: 00007f13b25e5fa0 R15: 00007ffe4c664f58
 </TASK>
----------------
Code disassembly (best guess):
   0:	85 e4                	test   %esp,%esp
   2:	75 07                	jne    0xb
   4:	e8 33 4f f5 fd       	call   0xfdf54f3c
   9:	eb 7a                	jmp    0x85
   b:	e8 9c 28 db fd       	call   0xfddb28ac
  10:	84 c0                	test   %al,%al
  12:	74 20                	je     0x34
  14:	e8 23 4f f5 fd       	call   0xfdf54f3c
  19:	eb 6a                	jmp    0x85
  1b:	e8 1c 4f f5 fd       	call   0xfdf54f3c
  20:	48 8d 3d 75 94 de 0b 	lea    0xbde9475(%rip),%rdi        # 0xbde949c
  27:	44 89 fe             	mov    %r15d,%esi
* 2a:	67 48 0f b9 3a       	ud1    (%edx),%rdi <-- trapping instruction
  2f:	e9 0b ff ff ff       	jmp    0xffffff3f
  34:	e8 93 46 91 07       	call   0x79146cc
  39:	41 89 c4             	mov    %eax,%r12d
  3c:	31 ff                	xor    %edi,%edi
  3e:	89 c6                	mov    %eax,%esi