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SubjectBUG: Dentry still in use [unmount of tmpfs tmpfs]
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Hello,

syzbot found the following crash on:

HEAD commit: 79c3ba32 Merge tag 'drm-fixes-2019-06-07-1' of git://anong..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=16eacf36a00000
kernel config: https://syzkaller.appspot.com/x/.config?x=60564cb52ab29d5b
dashboard link: https://syzkaller.appspot.com/bug?extid=7008b8b8ba7df475fdc8
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+7008b8b8ba7df475fdc8@syzkaller.appspotmail.com

BUG: Dentry 00000000c5e232d4{i=15d04,n=/} still in use (2) [unmount of
tmpfs tmpfs]
WARNING: CPU: 0 PID: 22126 at fs/dcache.c:1529 umount_check
fs/dcache.c:1520 [inline]
WARNING: CPU: 0 PID: 22126 at fs/dcache.c:1529 umount_check.cold+0xe9/0x10a
fs/dcache.c:1510
Kernel panic - not syncing: panic_on_warn set ...
CPU: 0 PID: 22126 Comm: syz-executor.5 Not tainted 5.2.0-rc3+ #16
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+0x2cb/0x744 kernel/panic.c:219
__warn.cold+0x20/0x4d 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+0x14/0x20 arch/x86/entry/entry_64.S:986
RIP: 0010:umount_check fs/dcache.c:1529 [inline]
RIP: 0010:umount_check.cold+0xe9/0x10a fs/dcache.c:1510
Code: 89 ff e8 50 62 f0 ff 48 81 c3 68 06 00 00 45 89 e8 4c 89 e1 53 4d 8b
0f 4c 89 f2 4c 89 e6 48 c7 c7 c0 ff 75 87 e8 31 d4 a1 ff <0f> 0b 58 e9 bd
2a ff ff e8 20 62 f0 ff e9 29 ff ff ff 45 31 f6 e9
RSP: 0018:ffff8880659d7bf8 EFLAGS: 00010286
RAX: 0000000000000054 RBX: ffff8880934748a8 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff815ac976 RDI: ffffed100cb3af71
RBP: ffff8880659d7c28 R08: 0000000000000054 R09: ffffed1015d06011
R10: ffffed1015d06010 R11: ffff8880ae830087 R12: ffff88808fa10840
R13: 0000000000000002 R14: 0000000000015d04 R15: ffffffff88c21260
d_walk+0x194/0x950 fs/dcache.c:1264
do_one_tree+0x28/0x40 fs/dcache.c:1536
shrink_dcache_for_umount+0x72/0x170 fs/dcache.c:1552
generic_shutdown_super+0x6d/0x370 fs/super.c:443
kill_anon_super+0x3e/0x60 fs/super.c:1137
kill_litter_super+0x50/0x60 fs/super.c:1146
deactivate_locked_super+0x95/0x100 fs/super.c:331
deactivate_super fs/super.c:362 [inline]
deactivate_super+0x1b2/0x1d0 fs/super.c:358
cleanup_mnt+0xbf/0x160 fs/namespace.c:1120
__cleanup_mnt+0x16/0x20 fs/namespace.c:1127
task_work_run+0x145/0x1c0 kernel/task_work.c:113
tracehook_notify_resume include/linux/tracehook.h:185 [inline]
exit_to_usermode_loop+0x273/0x2c0 arch/x86/entry/common.c:168
prepare_exit_to_usermode arch/x86/entry/common.c:199 [inline]
syscall_return_slowpath arch/x86/entry/common.c:279 [inline]
do_syscall_64+0x58e/0x680 arch/x86/entry/common.c:304
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x412f61
Code: 75 14 b8 03 00 00 00 0f 05 48 3d 01 f0 ff ff 0f 83 04 1b 00 00 c3 48
83 ec 08 e8 0a fc ff ff 48 89 04 24 b8 03 00 00 00 0f 05 <48> 8b 3c 24 48
89 c2 e8 53 fc ff ff 48 89 d0 48 83 c4 08 48 3d 01
RSP: 002b:00007fff7b2152c0 EFLAGS: 00000293 ORIG_RAX: 0000000000000003
RAX: 0000000000000000 RBX: 0000000000000006 RCX: 0000000000412f61
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000005
RBP: 0000000000000001 R08: 00000000ea5b01f8 R09: ffffffffffffffff
R10: 00007fff7b2153a0 R11: 0000000000000293 R12: 0000000000760d58
R13: 0000000000077aaa R14: 0000000000077ad7 R15: 000000000075bf2c
Kernel Offset: disabled
Rebooting in 86400 seconds..


---
This bug is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
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