From a933af2b82ea90a51a2600cf88cfcc5f2fba9804 Mon Sep 17 00:00:00 2001
From: John Kacur <jkacur@redhat.com>
Date: Thu, 18 Nov 2021 14:12:23 -0500
Subject: [PATCH 2/2] rt-tests: cyclicdeadline: Fix double mount of cgroups

If /sys/fs/cgroup exists and it is type cgroup2, cyclicdeadline mounts it a
second time as cgroup.

systemd is creating cgroup2 for logins and this can hang the machine and
not allow logins.

Fix this by:

If /sys/fs/cgroup exists, then use it for cyclicdeadline_test.

If it exists but the type is not recognized, exit with an error.
Do not simply mount it as type cgroup.

TODO: If the file doesn't exit but cgroups are supported in the kernel,
the file could be created and mounted.

Signed-off-by: John Kacur <jkacur@redhat.com>
---
 src/sched_deadline/cyclicdeadline.c | 39 +++++++++++++++++++++++------
 1 file changed, 32 insertions(+), 7 deletions(-)

diff --git a/src/sched_deadline/cyclicdeadline.c b/src/sched_deadline/cyclicdeadline.c
index 4860a40f5e6b..1f9a5df42d4f 100644
--- a/src/sched_deadline/cyclicdeadline.c
+++ b/src/sched_deadline/cyclicdeadline.c
@@ -358,6 +358,32 @@ static int mounted(const char *path, long magic)
 #define CGROUP_PATH "/sys/fs/cgroup"
 #define CPUSET_PATH CGROUP_PATH "/cpuset"
 
+/**
+ * cgroup_mounted - test if the path /sys/fs/cgroup exists
+ * and is a supported type
+ *
+ * Returns -1 if the path does not exist
+ * Returns 0 if the path exists but is not a cgroup type
+ * Returns 1 if the path exists and supports cgroups
+ */
+static int cgroup_mounted(void)
+{
+	int ret;
+
+	ret = mounted(CGROUP_PATH, TMPFS_MAGIC);
+	if (ret == -1)
+		return -1;	/* path doesn't exist */
+	if (ret == 1)
+		return 1;	/* tmpfs */
+	ret = mounted(CGROUP_PATH, CGROUP_SUPER_MAGIC);
+	if (ret == 1)
+		return 1;	/* cgroup v1 */
+	ret = mounted(CGROUP_PATH, CGROUP2_SUPER_MAGIC);
+	if (ret == 1)
+		return 1;
+	return 0;	/* path exists but type is not recognized */
+}
+
 static int open_cpuset(const char *path, const char *name)
 {
 	char buf[MAXPATH];
@@ -383,14 +409,13 @@ static int mount_cpuset(void)
 	int fd;
 
 	/* Check if cgroups is already mounted. */
-	ret = mounted(CGROUP_PATH, TMPFS_MAGIC);
-	if (ret < 0)
+	ret = cgroup_mounted();
+	if (ret < 0)	/* /sys/fs/cgroup doesn't exist */
 		return ret;
-	if (!ret) {
-		ret = mount("cgroup_root", CGROUP_PATH, "tmpfs", 0, NULL);
-		if (ret < 0)
-			return ret;
-	}
+
+	if (!ret) /* /sys/fs/cgroup exists, but we don't recognize the type */
+		return -1;
+
 	ret = stat(CPUSET_PATH, &st);
 	if (ret < 0) {
 		ret = mkdir(CPUSET_PATH, 0755);
-- 
2.31.1