tracing: Add an unreg_all() callback to trigger commands

Add a new unreg_all() callback that can be used to remove all
command-specific triggers from an event and arrange to have it called
whenever a trigger file is opened with O_TRUNC set.

Commands that don't want truncate semantics, or existing commands that
don't implement this function simply do nothing and their triggers
remain intact.

Link: http://lkml.kernel.org/r/2b7d62854d01f28c19185e1bbb8f826f385edfba.1449767187.git.tom.zanussi@linux.intel.com

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace.h

@@ -1298,8 +1298,8 @@ struct event_trigger_ops {
  *	it (filters make a trigger require access to the trace record
  *	but are not always present).
  *
- * All the methods below, except for @set_filter(), must be
- * implemented.
+ * All the methods below, except for @set_filter() and @unreg_all(),
+ * must be implemented.
  *
  * @func: The callback function responsible for parsing and
  *	registering the trigger written to the 'trigger' file by the
@@ -1324,6 +1324,10 @@ struct event_trigger_ops {
  *	This is usually implemented by the generic utility function
  *	@unregister_trigger() (see trace_event_triggers.c).
  *
+ * @unreg_all: An optional function called to remove all the triggers
+ *	from the list of triggers associated with the event.  Called
+ *	when a trigger file is opened in truncate mode.
+ *
  * @set_filter: An optional function called to parse and set a filter
  *	for the trigger.  If no @set_filter() method is set for the
  *	event command, filters set by the user for the command will be
@@ -1350,6 +1354,7 @@ struct event_command {
 					 struct event_trigger_ops *ops,
 					 struct event_trigger_data *data,
 					 struct trace_event_file *file);
+	void			(*unreg_all)(struct trace_event_file *file);
 	int			(*set_filter)(char *filter_str,
 					      struct event_trigger_data *data,
 					      struct trace_event_file *file);

kernel/trace/trace_events_trigger.c

@@ -193,6 +193,19 @@ static int event_trigger_regex_open(struct inode *inode, struct file *file)
 		return -ENODEV;
 	}
 
+	if ((file->f_mode & FMODE_WRITE) &&
+	    (file->f_flags & O_TRUNC)) {
+		struct trace_event_file *event_file;
+		struct event_command *p;
+
+		event_file = event_file_data(file);
+
+		list_for_each_entry(p, &trigger_commands, list) {
+			if (p->unreg_all)
+				p->unreg_all(event_file);
+		}
+	}
+
 	if (file->f_mode & FMODE_READ) {
 		ret = seq_open(file, &event_triggers_seq_ops);
 		if (!ret) {