Hi Stanislav,

I've looked more into it and I found some issues with the patch:

Stanislav Brabec <sbrabec@suse.cz> writes:
> +	while ((c = *subpath++))
                      ^^^ subpath is always post-incremented
> +	{
> +		if (c == '/') {
> +			if (!last_is_slash) {
> +				component_no++;
> +				if (component_no == 3)
> +					break;
                                         ^^^ when we break subpath
                                             actually 1 past the /
> +			}
> +			last_is_slash = true;
> +		} else
> +			last_is_slash = false;
> +	}
> +	if (component_no == 3) {
> +		int subpath_len = strlen(subpath);
> +		if (strncmp(root, subpath, subpath_len)) {
                             ^^^  ^^^ hence why here
with //localhost/share/dir in fstab we never get here
p root    => $1 = 0x7f28aa9977d0 "/dir"
p subpath => $2 = 0x7f28aa9979a2 "dir"

> +			if (*(root + subpath_len + 1) == 0)
> +				return "/";
> +			return root + subpath_len + 1;
> +		}
> +	}
> +	DBG(FS, ul_debugobj(fs, "cifs: leading part of root \"%s\" does
> not equal to mounted source subdir \"%s\"; should not happen", root,
> src));
^^^ this message gets printed with //localhost/share //localhost/share/ 

That being said, I could not make your patch fail i.e. make mount -a
mount the fs twice. So I'm not sure what happens there... I thought I
would give it a try.

I'm attaching a simpler patch that makes uses of streq_paths() in
lib/strutils.c (Note: streq_paths() and next_path_segment() have to be
backported for older versions of util-linux).