kernel panic during umount

[tohosys]

Dear @slavaim,

Could you please give me some help?
There is kernel panic if umount without deleting path.

test step:

1. insmod roflt
2. rfsctl -f roflt -i /test1
3. mount /dev/sdxx /test1
4. umount /dev/sdxx

dump stack:
#0 [ffff98e3f2ccfac0] machine_kexec at ffffffffaf063674
#1 [ffff98e3f2ccfb20] __crash_kexec at ffffffffaf11cf32
#2 [ffff98e3f2ccfbf0] crash_kexec at ffffffffaf11d020
#3 [ffff98e3f2ccfc08] oops_end at ffffffffaf76d758
#4 [ffff98e3f2ccfc30] die at ffffffffaf02f95b
#5 [ffff98e3f2ccfc60] do_trap at ffffffffaf76cea0
#6 [ffff98e3f2ccfcb0] do_invalid_op at ffffffffaf02c2a4
#7 [ffff98e3f2ccfd60] invalid_op at ffffffffaf77912e
[exception RIP: shrink_dcache_for_umount_subtree+460]
RIP: ffffffffaf25a9bc RSP: ffff98e3f2ccfe10 RFLAGS: 00010246
RAX: 000000000000004d RBX: ffff98e354797680 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffff98e49f013898 RDI: ffff98e49f013898
RBP: ffff98e3f2ccfe30 R8: 6e6544203a475542 R9: 66207972746e6544
R10: 0000000000000833 R11: 3533653839666666 R12: ffff98e354797680
R13: 00000000000003f3 R14: ffffffffb00e65b0 R15: ffff98e3e1a85910
ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018
#8 [ffff98e3f2ccfe38] shrink_dcache_for_umount at ffffffffaf25c2ef
#9 [ffff98e3f2ccfe50] generic_shutdown_super at ffffffffaf24416f
#10 [ffff98e3f2ccfe70] kill_block_super at ffffffffaf244627
#11 [ffff98e3f2ccfe90] deactivate_locked_super at ffffffffaf24496e
#12 [ffff98e3f2ccfeb0] deactivate_super at ffffffffaf2450f6
#13 [ffff98e3f2ccfec8] cleanup_mnt at ffffffffaf26366f
#14 [ffff98e3f2ccfee0] __cleanup_mnt at ffffffffaf263702
#15 [ffff98e3f2ccfef0] task_work_run at ffffffffaf0be7db
#16 [ffff98e3f2ccff30] do_notify_resume at ffffffffaf02bc65
#17 [ffff98e3f2ccff50] int_signal at ffffffffaf776124

Notes:

1. If i delete path by "rfsctl -f roflt -c" before umount then no problem.
2. If i hold some mounted file like "tail -f /test1/locktest" and umount by "umount -l /dev/sdxx" wirh lazy option then no problem.
3. OS is CentOS 7.

thanks.

[tohosys]

workaround:

• hook and hold umount system call
• clear redirfs filter's paths
• re-run umount system call