[PATCH RESEND] tracing: Make tracing work when debugfs is not compiled or initialized.

[Jiaxing Wang <hello.wjx@gmail.com>]

Currently tracing_init_dentry() returns -ENODEV when debugfs is not
initialized, which causes tracefs not populated with tracing files and
directories, so we will get an empty directory even after we manually
mount tracefs.

We can make tracing_init_dentry() return NULL as long as tracefs
is initialized and get a populated tracefs.

We also need to make global_trace.dir not NULL in order to pass the checks
in tracing_get_dentry() and add_tracer_options().

Also added stub debugfs_create_automount() for when debugfs is not
configured in.

Signed-off-by: Jiaxing Wang <hello.wjx@gmail.com>
---
 include/linux/debugfs.h |  8 ++++++++
 kernel/trace/Kconfig    |  1 -
 kernel/trace/trace.c    | 29 +++++++++++++++++------------
 3 files changed, 25 insertions(+), 13 deletions(-)

diff --git a/include/linux/debugfs.h b/include/linux/debugfs.h
index 9beb636..b42ef88 100644
--- a/include/linux/debugfs.h
+++ b/include/linux/debugfs.h
@@ -160,6 +160,14 @@ static inline struct dentry *debugfs_create_symlink(const char *name,
 	return ERR_PTR(-ENODEV);
 }
 
+static inline struct dentry *debugfs_create_automount(const char *name,
+					struct dentry *parent,
+					struct vfsmount *(*f)(void *),
+					void *data)
+{
+	return ERR_PTR(-ENODEV);
+}
+
 static inline void debugfs_remove(struct dentry *dentry)
 { }
 
diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index 1153c43..59f6377f 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -95,7 +95,6 @@ config RING_BUFFER_ALLOW_SWAP
 
 config TRACING
 	bool
-	select DEBUG_FS
 	select RING_BUFFER
 	select STACKTRACE if STACKTRACE_SUPPORT
 	select TRACEPOINTS
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 6e79408..6dd064e 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -6706,6 +6706,8 @@ static struct vfsmount *trace_automount(void *ingore)
 	return mnt;
 }
 
+#define TRACE_TOP_DIR_ENTRY	((struct dentry *)1)
+
 /**
  * tracing_init_dentry - initialize top level trace array
  *
@@ -6716,27 +6718,30 @@ static struct vfsmount *trace_automount(void *ingore)
 struct dentry *tracing_init_dentry(void)
 {
 	struct trace_array *tr = &global_trace;
+	struct dentry *traced;
 
 	/* The top level trace array uses  NULL as parent */
 	if (tr->dir)
 		return NULL;
 
-	if (WARN_ON(!debugfs_initialized()))
+	if (WARN_ON(!tracefs_initialized()))
 		return ERR_PTR(-ENODEV);
 
-	/*
-	 * 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);
+	if (debugfs_initialized()) {
+		/*
+		 * 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.
+		 */
+		traced = debugfs_create_automount("tracing", NULL,
+						   trace_automount, NULL);
+		if (!traced)
+			pr_warn_once("Could not create debugfs directory 'tracing'\n");
 	}
 
+	tr->dir = TRACE_TOP_DIR_ENTRY;
+
 	return NULL;
 }
 
-- 
2.1.4