Fix race condition in create_v2_cgroup

There used to be a race condition while creating the cgroup hierarchy:
if a parent directory was created after .exists() but before
.create_dir, the function failed unnecessarily.

This commit changes the function to use create_dir_all, which is the
atomic variant of the above pattern, and fixes some surrounding
docstrings on the way.

Signed-off-by: Markus Rudy <mr@edgeless.systems>

src/fs/cgroup.rs

@@ -550,18 +550,18 @@ fn create_v2_cgroup(
     // path: "a/b/c"
     let elements = path.split('/').collect::<Vec<&str>>();
     let last_index = elements.len() - 1;
+    // Build up the directory hierarchy element by element, enabling the controllers for all
+    // parents along the way.
     for (i, ele) in elements.iter().enumerate() {
         // ROOT/a
         fp.push(ele);
-        // create dir, need not check if is a file or directory
-        if !fp.exists() {
-            if let Err(e) = std::fs::create_dir(fp.clone()) {
+        // create dir if necessary
+        if let Err(e) = std::fs::create_dir_all(fp.clone()) {
             return Err(Error::with_cause(ErrorKind::FsError, e));
         }
-        }
 
         if i < last_index {
-            // enable controllers for substree
+            // enable controllers for subtree
             enable_controllers(&controllers, &fp);
         }
     }