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SubjectWARNING in close_fs_devices

syzbot found the following crash on:

HEAD commit: 716a685fdb89 Merge branch 'x86-hyperv-for-linus' of git://..
git tree: upstream
console output:
kernel config:
dashboard link:
compiler: gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller repro:
C reproducer:

IMPORTANT: if you fix the bug, please add the following tag to the commit:

WARNING: CPU: 1 PID: 4499 at fs/btrfs/volumes.c:1071
close_fs_devices+0xbc7/0xfa0 fs/btrfs/volumes.c:1071
Kernel panic - not syncing: panic_on_warn set ...

CPU: 1 PID: 4499 Comm: syz-executor921 Not tainted 4.17.0+ #84
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x1b9/0x294 lib/dump_stack.c:113
panic+0x22f/0x4de kernel/panic.c:184
__warn.cold.8+0x163/0x1b3 kernel/panic.c:536
report_bug+0x252/0x2d0 lib/bug.c:186
fixup_bug arch/x86/kernel/traps.c:178 [inline]
do_error_trap+0x1fc/0x4d0 arch/x86/kernel/traps.c:296
do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:316
invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:992
RIP: 0010:close_fs_devices+0xbc7/0xfa0 fs/btrfs/volumes.c:1071
Code: a0 b4 fe 0f 0b e8 89 a0 b4 fe 0f 0b 48 89 f7 e8 8f 10 f1 fe e9 f6 f5
ff ff e8 75 a0 b4 fe 0f 0b e9 fd fd ff ff e8 69 a0 b4 fe <0f> 0b e9 a8 fd
ff ff 48 8b bd 58 fd ff ff e8 26 11 f1 fe e9 35 fd
RSP: 0018:ffff8801ad45e530 EFLAGS: 00010293
RAX: ffff8801adb96200 RBX: 0000000000000001 RCX: ffffffff82c4ae3d
RDX: 0000000000000000 RSI: ffffffff82c4b097 RDI: 0000000000000007
RBP: ffff8801ad45e830 R08: ffff8801adb96200 R09: 0000000000000006
R10: ffff8801adb96200 R11: 0000000000000000 R12: ffffed0035a8bcc1
R13: 1ffff10035a8bcf9 R14: ffff8801ad45e7c8 R15: ffff8801b4431688
btrfs_close_devices+0x29/0x150 fs/btrfs/volumes.c:1085
open_ctree+0x589/0x7898 fs/btrfs/disk-io.c:3358
btrfs_fill_super fs/btrfs/super.c:1202 [inline]
btrfs_mount_root+0x16df/0x1e70 fs/btrfs/super.c:1593
mount_fs+0xae/0x328 fs/super.c:1277
vfs_kern_mount.part.34+0xd4/0x4d0 fs/namespace.c:1037
vfs_kern_mount+0x40/0x60 fs/namespace.c:1027
btrfs_mount+0x4a1/0x213e fs/btrfs/super.c:1661
mount_fs+0xae/0x328 fs/super.c:1277
vfs_kern_mount.part.34+0xd4/0x4d0 fs/namespace.c:1037
vfs_kern_mount fs/namespace.c:1027 [inline]
do_new_mount fs/namespace.c:2518 [inline]
do_mount+0x564/0x30b0 fs/namespace.c:2848
ksys_mount+0x12d/0x140 fs/namespace.c:3064
__do_sys_mount fs/namespace.c:3078 [inline]
__se_sys_mount fs/namespace.c:3075 [inline]
__x64_sys_mount+0xbe/0x150 fs/namespace.c:3075
do_syscall_64+0x1b1/0x800 arch/x86/entry/common.c:287
RIP: 0033:0x4431da
Code: b8 08 00 00 00 0f 05 48 3d 01 f0 ff ff 0f 83 fd e5 fb ff c3 66 2e 0f
1f 84 00 00 00 00 00 66 90 49 89 ca b8 a5 00 00 00 0f 05 <48> 3d 01 f0 ff
ff 0f 83 da e5 fb ff c3 66 0f 1f 84 00 00 00 00 00
RSP: 002b:00007ffe426bfd28 EFLAGS: 00000286 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00000000004431da
RDX: 0000000020000000 RSI: 0000000020000100 RDI: 00007ffe426bfd40
RBP: 0000000000000004 R08: 00000000200006c0 R09: 000000000000000a
R10: 0000000000000001 R11: 0000000000000286 R12: 0000000000000010
R13: 0000000008100000 R14: 0031656c69662f2e R15: fe03f80fe03f80ff
Dumping ftrace buffer:
(ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..

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