Removed USB storage device is still mounted

Question

I'm working on a embedded linux system based on busybox. Sometimes a USB device which is physically removed from the post is still displayed in mount, df or /proc/mounts. If I reattach the device, it is detected as for example /dev/sdb1 while the /dev/sda1 device is still mounted.

I know that open file handles on removed devices can cause this but is there any other condition which can trigger this bahavior?

Answer 1

You could write a udev rule like

ACTION=="remove", KERNEL=="sd[b-z][0-9]", SUBSYSTEM=="block", RUN+="umount-removed.sh"

Which will trigger the unplug event and then umount the file system from that script

MNT_POINT=`df | grep "$DEVNAME" | awk '{print $6}'`

#If still mounted
if [ ! -z "$MNT_POINT" ];then
    logger -i -t usbrm -p daemon.notice "The device $DEVNAME is still mounted"
    #Umount fs
    umount $DEVNAME

    if [ $? -eq 0 ];then
        logger -i -t usbrm -p daemon.notice "$DEVNAME successfully unmounted from $MNT_POINT"
    else
        logger -i -t usbrm -p daemon.err "Impossible to umount $DEVNAME from $MNT_POINT. Aborting..."
    fi
fi

You can see the log in /var/daemon.log.

Edit : Forgot to precise that udev use a specific name space so you have to configure it to share mount point with userspace.

To do it locate the file systemd-udevd.service on your system and duplicate it to /etc/systemd/system/systemd-udevd.service and replace content to MountFlags=shared