Change in behaviour when unmounting recursive bind mounts

[Tvrtko Ursulin <tvrtko.ursulin@onelan.co.uk>]

Hi Al, all,

Please have a look at the command sequence below, does it look right to you?

+ M1=testmp1
+ M2=testmp2
+ SM=submount
+ mkdir -p testmp1
+ mkdir -p testmp2
+ mount none -t tmpfs testmp1
+ mkdir -p testmp1/submount
+ mount none -t tmpfs testmp1/submount
+ strace -f -e trace=mount,umount mount testmp1 --rbind testmp2
mount("/dev/shm/testmp1", "/dev/shm/testmp2", 0x7fcd30b821e0, MS_MGC_VAL|
MS_BIND|MS_REC, NULL) = 0
+++ exited with 0 +++
+ strace -f -e trace=mount,umount umount -l testmp2
umount("/dev/shm/testmp2", MNT_DETACH)  = 0
+++ exited with 0 +++
+ mountpoint testmp1/submount
testmp1/submount is not a mountpoint
+ echo nay
nay
+ umount testmp2
umount: testmp2: not mounted
+ umount testmp1
+ rmdir testmp1
+ rmdir testmp2
rmdir: failed to remove `testmp2': Device or resource busy

Previously unmounting the recursive bind target would not unmount the source, 
which to me looks like a more sensible outcome. 

However what could be causing this weird behaviour puzzles me since I am not 
sure it is purely the kernel on it's own. When I first hit this problem some 
months ago unfortunately I did not have the time to dig deeper. However a 
mental note remained that I thought, at the time, it is possible some 
userspace change was a contributing factor.

Regards,

Tvrtko