tracing: Add interface to allow multiple trace buffers

Add the interface ("instances" directory) to add multiple buffers
to ftrace. To create a new instance, simply do a mkdir in the
instances directory:

This will create a directory with the following:

 # cd instances
 # mkdir foo
 # ls foo
buffer_size_kb        free_buffer  trace_clock    trace_pipe
buffer_total_size_kb  set_event    trace_marker   tracing_enabled
events/               trace        trace_options  tracing_on

Currently only events are able to be set, and there isn't a way
to delete a buffer when one is created (yet).

Note, the i_mutex lock is dropped from the parent "instances"
directory during the mkdir operation. As the "instances" directory
can not be renamed or deleted (created on boot), I do not see
any harm in dropping the lock. The creation of the sub directories
is protected by trace_types_lock mutex, which only lets one
instance get into the code path at a time. If two tasks try to
create or delete directories of the same name, only one will occur
and the other will fail with -EEXIST.

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

kernel/trace/trace_events.c

@@ -1754,16 +1754,22 @@ int event_trace_add_tracer(struct dentry *parent, struct trace_array *tr)
 	struct dentry *d_events;
 	struct dentry *entry;
 
+	mutex_lock(&event_mutex);
+
 	entry = debugfs_create_file("set_event", 0644, parent,
 				    tr, &ftrace_set_event_fops);
 	if (!entry) {
 		pr_warning("Could not create debugfs 'set_event' entry\n");
+		mutex_unlock(&event_mutex);
 		return -ENOMEM;
 	}
 
 	d_events = debugfs_create_dir("events", parent);
-	if (!d_events)
+	if (!d_events) {
 		pr_warning("Could not create debugfs 'events' directory\n");
+		mutex_unlock(&event_mutex);
+		return -ENOMEM;
+	}
 
 	/* ring buffer internal formats */
 	trace_create_file("header_page", 0444, d_events,
@@ -1778,7 +1784,11 @@ int event_trace_add_tracer(struct dentry *parent, struct trace_array *tr)
 			  tr, &ftrace_tr_enable_fops);
 
 	tr->event_dir = d_events;
+	down_write(&trace_event_mutex);
 	__trace_add_event_dirs(tr);
+	up_write(&trace_event_mutex);
+
+	mutex_unlock(&event_mutex);
 
 	return 0;
 }