tracing: Automatically mount tracefs on debugfs/tracing

As tools currently rely on the tracing directory in debugfs, we can not
just created a tracefs infrastructure and expect sysadmins to mount
the new tracefs to have their old tools work.

Instead, the debugfs tracing directory is still created and the tracefs
file system is mounted there when the debugfs filesystem is mounted.

No longer does the tracing infrastructure update the debugfs file system,
but instead interacts with the tracefs file system. But now, it still
appears to the user like nothing changed, except you also have the feature
of mounting just the tracing system without needing all of debugfs!

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace.c

@@ -32,6 +32,7 @@
 #include <linux/splice.h>
 #include <linux/kdebug.h>
 #include <linux/string.h>
+#include <linux/mount.h>
 #include <linux/rwsem.h>
 #include <linux/slab.h>
 #include <linux/ctype.h>
@@ -6535,6 +6536,28 @@ init_tracer_tracefs(struct trace_array *tr, struct dentry *d_tracer)
 
 }
 
+static struct vfsmount *trace_automount(void *ingore)
+{
+	struct vfsmount *mnt;
+	struct file_system_type *type;
+
+	/*
+	 * To maintain backward compatibility for tools that mount
+	 * debugfs to get to the tracing facility, tracefs is automatically
+	 * mounted to the debugfs/tracing directory.
+	 */
+	type = get_fs_type("tracefs");
+	if (!type)
+		return NULL;
+	mnt = vfs_kern_mount(type, 0, "tracefs", NULL);
+	put_filesystem(type);
+	if (IS_ERR(mnt))
+		return NULL;
+	mntget(mnt);
+
+	return mnt;
+}
+
 /**
  * tracing_init_dentry - initialize top level trace array
  *
@@ -6546,14 +6569,21 @@ struct dentry *tracing_init_dentry(void)
 {
 	struct trace_array *tr = &global_trace;
 
+	/* The top level trace array uses  NULL as parent */
 	if (tr->dir)
-		return tr->dir;
+		return NULL;
 
 	if (WARN_ON(!debugfs_initialized()))
 		return ERR_PTR(-ENODEV);
 
-	tr->dir = debugfs_create_dir("tracing", NULL);
-
+	/*
+	 * As there may still be users that expect the tracing
+	 * files to exist in debugfs/tracing, we must automount
+	 * the tracefs file system there, so older tools still
+	 * work with the newer kerenl.
+	 */
+	tr->dir = debugfs_create_automount("tracing", NULL,
+					   trace_automount, NULL);
 	if (!tr->dir) {
 		pr_warn_once("Could not create debugfs directory 'tracing'\n");
 		return ERR_PTR(-ENOMEM);