tools api fs: Prefer cgroup v1 path in cgroupfs_find_mountpoint()

The cgroupfs_find_mountpoint() looks up the /proc/mounts file to find
a directory for the given cgroup subsystem.  It keeps both cgroup v1
and v2 path since there's a possibility of the mixed hierarchly.

But we can simply use v1 path if it's found as it will override the v2
hierarchy.
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20201216090556.813996-1-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/api/fs/cgroup.c

@@ -12,7 +12,7 @@ int cgroupfs_find_mountpoint(char *buf, size_t maxlen, const char *subsys)
 {
 	FILE *fp;
 	char mountpoint[PATH_MAX + 1], tokens[PATH_MAX + 1], type[PATH_MAX + 1];
-	char path_v1[PATH_MAX + 1], path_v2[PATH_MAX + 2], *path;
+	char path_v2[PATH_MAX + 1];
 	char *token, *saved_ptr = NULL;
 
 	fp = fopen("/proc/mounts", "r");
@@ -22,45 +22,41 @@ int cgroupfs_find_mountpoint(char *buf, size_t maxlen, const char *subsys)
 	/*
 	 * in order to handle split hierarchy, we need to scan /proc/mounts
 	 * and inspect every cgroupfs mount point to find one that has
-	 * perf_event subsystem
+	 * the given subsystem.  If we found v1, just use it.  If not we can
+	 * use v2 path as a fallback.
 	 */
-	path_v1[0] = '\0';
 	path_v2[0] = '\0';
 
 	while (fscanf(fp, "%*s %"__stringify(PATH_MAX)"s %"__stringify(PATH_MAX)"s %"
 				__stringify(PATH_MAX)"s %*d %*d\n",
 				mountpoint, type, tokens) == 3) {
 
-		if (!path_v1[0] && !strcmp(type, "cgroup")) {
+		if (!strcmp(type, "cgroup")) {
 
 			token = strtok_r(tokens, ",", &saved_ptr);
 
 			while (token != NULL) {
 				if (subsys && !strcmp(token, subsys)) {
-					strcpy(path_v1, mountpoint);
-					break;
+					/* found */
+					fclose(fp);
+
+					if (strlen(mountpoint) < maxlen) {
+						strcpy(buf, mountpoint);
+						return 0;
+					}
+					return -1;
 				}
 				token = strtok_r(NULL, ",", &saved_ptr);
 			}
 		}
 
-		if (!path_v2[0] && !strcmp(type, "cgroup2"))
+		if (!strcmp(type, "cgroup2"))
 			strcpy(path_v2, mountpoint);
-
-		if (path_v1[0] && path_v2[0])
-			break;
 	}
 	fclose(fp);
 
-	if (path_v1[0])
-		path = path_v1;
-	else if (path_v2[0])
-		path = path_v2;
-	else
-		return -1;
-
-	if (strlen(path) < maxlen) {
-		strcpy(buf, path);
+	if (path_v2[0] && strlen(path_v2) < maxlen) {
+		strcpy(buf, path_v2);
 		return 0;
 	}
 	return -1;