umount /dev/shm

[root@test ~]# umount /dev/shm
umount: /dev/shm: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
[root@test ~]#  lsof |grep /dev/shm
gnome-set  2140      root  mem       REG               0,15  67108904      14588 /dev/shm/pulse-shm-2262568528
gnome-vol  2205      root  mem       REG               0,15  67108904      15465 /dev/shm/pulse-shm-1929015898
pulseaudi  2231      root  mem       REG               0,15  67108904      15478 /dev/shm/pulse-shm-1569886742
[root@test ~]#  kill -9 2140
[root@test ~]#  lsof |grep /dev/shm
gnome-vol  2205      root  mem       REG               0,15  67108904      15465 /dev/shm/pulse-shm-1929015898
pulseaudi  2231      root  mem       REG               0,15  67108904      15478 /dev/shm/pulse-shm-1569886742
[root@test ~]# kill -9 2205
[root@test ~]#  lsof |grep /dev/shm
pulseaudi  2231      root  mem       REG               0,15  67108904      15478 /dev/shm/pulse-shm-1569886742
[root@test ~]# kill -9 2231
[root@test ~]# umount /dev/shm
[root@test ~]# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/vg_test-lv_root
                       18G   12G  4.6G  73% /
/dev/sda1             477M   33M  419M   8% /boot