WARNING in cleanup_mnt

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    48b161983ae5 Merge tag 'xarray-5.0-rc3' of git://git.infra..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=10cc68ef400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=505743eba4e4f68
dashboard link: https://syzkaller.appspot.com/bug?extid=3f12aa66b747ba56fcae
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)

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

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+3f12aa66b747ba56fcae@xxxxxxxxxxxxxxxxxxxxxxxxx

Enabling of bearer <udp:syz0> rejected, already enabled
WARNING: CPU: 0 PID: 17831 at fs/namespace.c:1091 cleanup_mnt+0x118/0x160  
fs/namespace.c:1091
Kernel panic - not syncing: panic_on_warn set ...
Enabling of bearer <udp:syz0> rejected, already enabled
CPU: 0 PID: 17831 Comm: kworker/0:1 Not tainted 5.0.0-rc3+ #37
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Workqueue: events proc_cleanup_work
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1db/0x2d0 lib/dump_stack.c:113
 panic+0x2cb/0x65c kernel/panic.c:214
kobject: 'loop4' (00000000cd062423): kobject_uevent_env
 __warn.cold+0x20/0x48 kernel/panic.c:571
 report_bug+0x263/0x2b0 lib/bug.c:186
kobject: 'loop4' (00000000cd062423): fill_kobj_path: path  
= '/devices/virtual/block/loop4'
 fixup_bug arch/x86/kernel/traps.c:178 [inline]
 fixup_bug arch/x86/kernel/traps.c:173 [inline]
 do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:271
 do_invalid_op+0x37/0x50 arch/x86/kernel/traps.c:290
 invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:973
RIP: 0010:cleanup_mnt+0x118/0x160 fs/namespace.c:1091
Code: 1c 01 00 00 48 c7 c7 c0 c9 a5 89 e8 12 4e 11 06 48 8d 7b 38 48 c7 c6  
a0 0f db 81 e8 e2 77 91 ff 5b 41 5c 5d c3 e8 a8 dc a6 ff <0f> 0b e9 0e ff  
ff ff e8 9c dc a6 ff 48 89 df e8 94 fe 06 00 e9 31
RSP: 0018:ffff8880586b7888 EFLAGS: 00010293
RAX: ffff88809f4b4140 RBX: ffff8880909c3a00 RCX: ffffffff81db2334
RDX: 0000000000000000 RSI: ffffffff81db2428 RDI: 0000000000000005
RBP: ffff8880586b7898 R08: ffff88809f4b4140 R09: fffffbfff13024e6
R10: fffffbfff13024e5 R11: ffffffff8981272b R12: 00000000000000fc
R13: 0000000000004000 R14: dffffc0000000000 R15: ffff8880909c3a30
 mntput_no_expire fs/namespace.c:1175 [inline]
 mntput_no_expire+0x866/0xbd0 fs/namespace.c:1117
kobject: 'loop2' (00000000d5abe9f2): kobject_uevent_env
kobject: 'loop2' (00000000d5abe9f2): fill_kobj_path: path  
= '/devices/virtual/block/loop2'
 mntput fs/namespace.c:1185 [inline]
 create_mnt_ns fs/namespace.c:3005 [inline]
 kern_unmount+0xbd/0xf0 fs/namespace.c:2995
 pid_ns_release_proc+0x37/0x41 fs/proc/root.c:224
 proc_cleanup_work+0x19/0x20 kernel/pid_namespace.c:64
Enabling of bearer <udp:syz0> rejected, already enabled
 process_one_work+0xd0c/0x1ce0 kernel/workqueue.c:2153
Enabling of bearer <udp:syz0> rejected, already enabled
 worker_thread+0x143/0x14a0 kernel/workqueue.c:2296
 kthread+0x357/0x430 kernel/kthread.c:246
 ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:352
Kernel Offset: disabled
Rebooting in 86400 seconds..


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