BUG: Dentry still in use [unmount of nfsd nfsd]

BUG: Dentry still in use [unmount of nfsd nfsd]

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    f4eb1423 Merge branch 'for-linus' of git://git.kernel.org/..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11ab1c74600000
kernel config:  https://syzkaller.appspot.com/x/.config?x=a4c9e9f08e9e8960
dashboard link: https://syzkaller.appspot.com/bug?extid=2c95195d5d433f6ed6cb
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+2c95195d5d433f6ed6cb@syzkaller.appspotmail.com

RBP: 000000000075bf20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007fa0af7a86d4
R13: 00000000004c018f R14: 00000000004d2228 R15: 0000000000000004
BUG: Dentry 00000000117d3c54{i=0,n=clients}  still in use (1) [unmount of  
nfsd nfsd]
------------[ cut here ]------------
WARNING: CPU: 1 PID: 18817 at fs/dcache.c:1595 umount_check  
fs/dcache.c:1595 [inline]
WARNING: CPU: 1 PID: 18817 at fs/dcache.c:1595 umount_check.cold+0xf5/0x116  
fs/dcache.c:1576
Kernel panic - not syncing: panic_on_warn set ...
CPU: 1 PID: 18817 Comm: syz-executor.3 Not tainted 5.3.0-rc3+ #100
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+0x172/0x1f0 lib/dump_stack.c:113
  panic+0x2dc/0x755 kernel/panic.c:219
  __warn.cold+0x20/0x4c kernel/panic.c:576
  report_bug+0x263/0x2b0 lib/bug.c:186
  fixup_bug arch/x86/kernel/traps.c:179 [inline]
  fixup_bug arch/x86/kernel/traps.c:174 [inline]
  do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:272
  do_invalid_op+0x37/0x50 arch/x86/kernel/traps.c:291
  invalid_op+0x23/0x30 arch/x86/entry/entry_64.S:1028
RIP: 0010:umount_check fs/dcache.c:1595 [inline]
RIP: 0010:umount_check.cold+0xf5/0x116 fs/dcache.c:1576
Code: 00 00 45 89 e8 4c 89 e1 53 4d 8b 0f 4c 89 f2 4c 89 e6 48 c7 c7 80 97  
96 87 e8 61 a6 9f ff 48 c7 c7 00 98 96 87 e8 55 a6 9f ff <0f> 0b 58 e9 b1  
15 ff ff e8 24 1f f0 ff e9 1d ff ff ff 45 31 f6 e9
RSP: 0018:ffff888060187b88 EFLAGS: 00010282
RAX: 0000000000000024 RBX: ffff8880624f8bb8 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff815c3ba6 RDI: ffffed100c030f63
RBP: ffff888060187bb8 R08: 0000000000000024 R09: fffffbfff11b42c5
R10: fffffbfff11b42c4 R11: ffffffff88da1623 R12: ffff88803f50fa20
R13: 0000000000000001 R14: 0000000000000000 R15: ffffffff88f75220
  d_walk+0x283/0x950 fs/dcache.c:1305
  do_one_tree+0x28/0x40 fs/dcache.c:1602
  shrink_dcache_for_umount+0x72/0x170 fs/dcache.c:1618
  generic_shutdown_super+0x6d/0x370 fs/super.c:443
  kill_anon_super+0x3e/0x60 fs/super.c:1102
  kill_litter_super+0x50/0x60 fs/super.c:1111
  nfsd_umount+0x3f/0x90 fs/nfsd/nfsctl.c:1416
  deactivate_locked_super+0x95/0x100 fs/super.c:331
  vfs_get_super+0x210/0x270 fs/super.c:1185
  nfsd_fs_get_tree+0x7a/0x90 fs/nfsd/nfsctl.c:1390
  vfs_get_tree+0x8e/0x390 fs/super.c:1413
  vfs_fsconfig_locked+0x236/0x3d0 fs/fsopen.c:232
  __do_sys_fsconfig fs/fsopen.c:445 [inline]
  __se_sys_fsconfig fs/fsopen.c:314 [inline]
  __x64_sys_fsconfig+0x8e0/0xa40 fs/fsopen.c:314
  do_syscall_64+0xfd/0x6a0 arch/x86/entry/common.c:296
  entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x459829
Code: fd b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 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 0f 83 cb b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fa0af7a7c78 EFLAGS: 00000246 ORIG_RAX: 00000000000001af
RAX: ffffffffffffffda RBX: 00007fa0af7a7c90 RCX: 0000000000459829
RDX: 0000000000000000 RSI: 0000000000000006 RDI: 0000000000000003
RBP: 000000000075bf20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007fa0af7a86d4
R13: 00000000004c018f R14: 00000000004d2228 R15: 0000000000000004
Kernel Offset: disabled
Rebooting in 86400 seconds..


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