selftests/resctrl: Unmount resctrl FS before starting the first test

Resctrl FS mount/remount/umount code is hard to track. Better approach
is to use mount/umount pair for each test but that assumes resctrl FS
is not mounted beforehand.

Change umount_resctrlfs() so that it can unmount resctrl FS from any
path, and enable further simplifications into mount/remount/umount
logic by unmounting resctrl FS at the start if a pre-existing
mountpoint is found.
Suggested-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Tested-by: Babu Moger <babu.moger@amd.com>
Tested-by: Shaopeng Tan (Fujitsu) <tan.shaopeng@fujitsu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/resctrl/resctrl_tests.c

@@ -250,6 +250,9 @@ int main(int argc, char **argv)
 	if (!check_resctrlfs_support())
 		return ksft_exit_skip("resctrl FS does not exist. Enable X86_CPU_RESCTRL config option.\n");
 
+	if (umount_resctrlfs())
+		return ksft_exit_skip("resctrl FS unmount failed.\n");
+
 	filter_dmesg();
 
 	ksft_set_plan(tests ? : 4);

tools/testing/selftests/resctrl/resctrlfs.c

@@ -82,10 +82,16 @@ int remount_resctrlfs(bool mum_resctrlfs)
 
 int umount_resctrlfs(void)
 {
-	if (find_resctrl_mount(NULL))
+	char mountpoint[256];
+	int ret;
+
+	ret = find_resctrl_mount(mountpoint);
+	if (ret == -ENOENT)
 		return 0;
+	if (ret)
+		return ret;
 
-	if (umount(RESCTRL_PATH)) {
+	if (umount(mountpoint)) {
 		perror("# Unable to umount resctrl");
 
 		return errno;