[PATCH v9 22/37] tracing: Add variable reference handling to hist triggers

[Tom Zanussi]

Add the necessary infrastructure to allow the variables defined on one
event to be referenced in another.  This allows variables set by a
previous event to be referenced and used in expressions combining the
variable values saved by that previous event and the event fields of
the current event.  For example, here's how a latency can be
calculated and saved into yet another variable named 'wakeup_lat':

    # echo 'hist:keys=pid,prio:ts0=common_timestamp ...
    # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp-$ts0 ...

In the first event, the event's timetamp is saved into the variable
ts0.  In the next line, ts0 is subtracted from the second event's
timestamp to produce the latency.

Further users of variable references will be described in subsequent
patches, such as for instance how the 'wakeup_lat' variable above can
be displayed in a latency histogram.

Signed-off-by: Tom Zanussi <tom.zanussi@xxxxxxxxxxxxxxx>
---
 kernel/trace/trace.c                |   2 +
 kernel/trace/trace.h                |   3 +
 kernel/trace/trace_events_hist.c    | 661 +++++++++++++++++++++++++++++++++++-
 kernel/trace/trace_events_trigger.c |   6 +
 4 files changed, 656 insertions(+), 16 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 27b0f6c..4951484 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -7788,6 +7788,7 @@ static int instance_mkdir(const char *name)
 
 	INIT_LIST_HEAD(&tr->systems);
 	INIT_LIST_HEAD(&tr->events);
+	INIT_LIST_HEAD(&tr->hist_vars);
 
 	if (allocate_trace_buffers(tr, trace_buf_size) < 0)
 		goto out_free_tr;
@@ -8538,6 +8539,7 @@ __init static int tracer_alloc_buffers(void)
 
 	INIT_LIST_HEAD(&global_trace.systems);
 	INIT_LIST_HEAD(&global_trace.events);
+	INIT_LIST_HEAD(&global_trace.hist_vars);
 	list_add(&global_trace.list, &ftrace_trace_arrays);
 
 	apply_trace_boot_options();
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 89771b4..99b7ee7 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -274,6 +274,7 @@ struct trace_array {
 	int			function_enabled;
 #endif
 	int			time_stamp_abs_ref;
+	struct list_head	hist_vars;
 };
 
 enum {
@@ -1548,6 +1549,8 @@ extern int save_named_trigger(const char *name,
 extern void unpause_named_trigger(struct event_trigger_data *data);
 extern void set_named_trigger_data(struct event_trigger_data *data,
 				   struct event_trigger_data *named_data);
+extern struct event_trigger_data *
+get_named_trigger_data(struct event_trigger_data *data);
 extern int register_event_command(struct event_command *cmd);
 extern int unregister_event_command(struct event_command *cmd);
 extern int register_trigger_hist_enable_disable_cmds(void);
diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index a2800c2..25dd082 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -59,7 +59,12 @@ struct hist_field {
 	struct hist_trigger_data	*hist_data;
 	struct hist_var			var;
 	enum field_op_id		operator;
+	char				*system;
+	char				*event_name;
 	char				*name;
+	unsigned int			var_idx;
+	unsigned int			var_ref_idx;
+	bool                            read_once;
 };
 
 static u64 hist_field_none(struct hist_field *field,
@@ -214,6 +219,7 @@ enum hist_field_flags {
 	HIST_FIELD_FL_TIMESTAMP_USECS	= 1 << 11,
 	HIST_FIELD_FL_VAR		= 1 << 12,
 	HIST_FIELD_FL_EXPR		= 1 << 13,
+	HIST_FIELD_FL_VAR_REF		= 1 << 14,
 };
 
 struct var_defs {
@@ -253,6 +259,8 @@ struct hist_trigger_data {
 	struct tracing_map		*map;
 	bool				enable_timestamps;
 	bool				remove;
+	struct hist_field               *var_refs[TRACING_MAP_VARS_MAX];
+	unsigned int			n_var_refs;
 };
 
 static u64 hist_field_timestamp(struct hist_field *hist_field,
@@ -271,6 +279,214 @@ static u64 hist_field_timestamp(struct hist_field *hist_field,
 	return ts;
 }
 
+struct hist_var_data {
+	struct list_head list;
+	struct hist_trigger_data *hist_data;
+};
+
+static struct hist_field *
+check_field_for_var_ref(struct hist_field *hist_field,
+			struct hist_trigger_data *var_data,
+			unsigned int var_idx)
+{
+	struct hist_field *found = NULL;
+
+	if (hist_field && hist_field->flags & HIST_FIELD_FL_VAR_REF) {
+		if (hist_field->var.idx == var_idx &&
+		    hist_field->var.hist_data == var_data) {
+			found = hist_field;
+		}
+	}
+
+	return found;
+}
+
+static struct hist_field *
+check_field_for_var_refs(struct hist_trigger_data *hist_data,
+			 struct hist_field *hist_field,
+			 struct hist_trigger_data *var_data,
+			 unsigned int var_idx,
+			 unsigned int level)
+{
+	struct hist_field *found = NULL;
+	unsigned int i;
+
+	if (level > 3)
+		return found;
+
+	if (!hist_field)
+		return found;
+
+	found = check_field_for_var_ref(hist_field, var_data, var_idx);
+	if (found)
+		return found;
+
+	for (i = 0; i < HIST_FIELD_OPERANDS_MAX; i++) {
+		struct hist_field *operand;
+
+		operand = hist_field->operands[i];
+		found = check_field_for_var_refs(hist_data, operand, var_data,
+						 var_idx, level + 1);
+		if (found)
+			return found;
+	}
+
+	return found;
+}
+
+static struct hist_field *find_var_ref(struct hist_trigger_data *hist_data,
+				       struct hist_trigger_data *var_data,
+				       unsigned int var_idx)
+{
+	struct hist_field *hist_field, *found = NULL;
+	unsigned int i;
+
+	for_each_hist_field(i, hist_data) {
+		hist_field = hist_data->fields[i];
+		found = check_field_for_var_refs(hist_data, hist_field,
+						 var_data, var_idx, 0);
+		if (found)
+			return found;
+	}
+
+	return found;
+}
+
+static struct hist_field *find_any_var_ref(struct hist_trigger_data *hist_data,
+					   unsigned int var_idx)
+{
+	struct trace_array *tr = hist_data->event_file->tr;
+	struct hist_field *found = NULL;
+	struct hist_var_data *var_data;
+
+	list_for_each_entry(var_data, &tr->hist_vars, list) {
+		if (var_data->hist_data == hist_data)
+			continue;
+		found = find_var_ref(var_data->hist_data, hist_data, var_idx);
+		if (found)
+			break;
+	}
+
+	return found;
+}
+
+static bool check_var_refs(struct hist_trigger_data *hist_data)
+{
+	struct hist_field *field;
+	bool found = false;
+	int i;
+
+	for_each_hist_field(i, hist_data) {
+		field = hist_data->fields[i];
+		if (field && field->flags & HIST_FIELD_FL_VAR) {
+			if (find_any_var_ref(hist_data, field->var.idx)) {
+				found = true;
+				break;
+			}
+		}
+	}
+
+	return found;
+}
+
+static struct hist_var_data *find_hist_vars(struct hist_trigger_data *hist_data)
+{
+	struct trace_array *tr = hist_data->event_file->tr;
+	struct hist_var_data *var_data, *found = NULL;
+
+	list_for_each_entry(var_data, &tr->hist_vars, list) {
+		if (var_data->hist_data == hist_data) {
+			found = var_data;
+			break;
+		}
+	}
+
+	return found;
+}
+
+static bool field_has_hist_vars(struct hist_field *hist_field,
+				unsigned int level)
+{
+	int i;
+
+	if (level > 3)
+		return false;
+
+	if (!hist_field)
+		return false;
+
+	if (hist_field->flags & HIST_FIELD_FL_VAR ||
+	    hist_field->flags & HIST_FIELD_FL_VAR_REF)
+		return true;
+
+	for (i = 0; i < HIST_FIELD_OPERANDS_MAX; i++) {
+		struct hist_field *operand;
+
+		operand = hist_field->operands[i];
+		if (field_has_hist_vars(operand, level + 1))
+			return true;
+	}
+
+	return false;
+}
+
+static bool has_hist_vars(struct hist_trigger_data *hist_data)
+{
+	struct hist_field *hist_field;
+	int i;
+
+	for_each_hist_field(i, hist_data) {
+		hist_field = hist_data->fields[i];
+		if (field_has_hist_vars(hist_field, 0))
+			return true;
+	}
+
+	return false;
+}
+
+static int save_hist_vars(struct hist_trigger_data *hist_data)
+{
+	struct trace_array *tr = hist_data->event_file->tr;
+	struct hist_var_data *var_data;
+
+	var_data = find_hist_vars(hist_data);
+	if (var_data)
+		return 0;
+
+	if (trace_array_get(tr) < 0)
+		return -ENODEV;
+
+	var_data = kzalloc(sizeof(*var_data), GFP_KERNEL);
+	if (!var_data) {
+		trace_array_put(tr);
+		return -ENOMEM;
+	}
+
+	var_data->hist_data = hist_data;
+	list_add(&var_data->list, &tr->hist_vars);
+
+	return 0;
+}
+
+static void remove_hist_vars(struct hist_trigger_data *hist_data)
+{
+	struct trace_array *tr = hist_data->event_file->tr;
+	struct hist_var_data *var_data;
+
+	var_data = find_hist_vars(hist_data);
+	if (!var_data)
+		return;
+
+	if (WARN_ON(check_var_refs(hist_data)))
+		return;
+
+	list_del(&var_data->list);
+
+	kfree(var_data);
+
+	trace_array_put(tr);
+}
+
 static struct hist_field *find_var_field(struct hist_trigger_data *hist_data,
 					 const char *var_name)
 {
@@ -313,10 +529,137 @@ static struct hist_field *find_var(struct hist_trigger_data *hist_data,
 	return NULL;
 }
 
+static struct trace_event_file *find_var_file(struct trace_array *tr,
+					      char *system,
+					      char *event_name,
+					      char *var_name)
+{
+	struct hist_trigger_data *var_hist_data;
+	struct hist_var_data *var_data;
+	struct trace_event_file *file, *found = NULL;
+
+	if (system)
+		return find_event_file(tr, system, event_name);
+
+	list_for_each_entry(var_data, &tr->hist_vars, list) {
+		var_hist_data = var_data->hist_data;
+		file = var_hist_data->event_file;
+		if (file == found)
+			continue;
+
+		if (find_var_field(var_hist_data, var_name)) {
+			if (found)
+				return NULL;
+
+			found = file;
+		}
+	}
+
+	return found;
+}
+
+static struct hist_field *find_file_var(struct trace_event_file *file,
+					const char *var_name)
+{
+	struct hist_trigger_data *test_data;
+	struct event_trigger_data *test;
+	struct hist_field *hist_field;
+
+	list_for_each_entry_rcu(test, &file->triggers, list) {
+		if (test->cmd_ops->trigger_type == ETT_EVENT_HIST) {
+			test_data = test->private_data;
+			hist_field = find_var_field(test_data, var_name);
+			if (hist_field)
+				return hist_field;
+		}
+	}
+
+	return NULL;
+}
+
+static struct hist_field *find_event_var(struct hist_trigger_data *hist_data,
+					 char *system,
+					 char *event_name,
+					 char *var_name)
+{
+	struct trace_array *tr = hist_data->event_file->tr;
+	struct hist_field *hist_field = NULL;
+	struct trace_event_file *file;
+
+	file = find_var_file(tr, system, event_name, var_name);
+	if (!file)
+		return NULL;
+
+	hist_field = find_file_var(file, var_name);
+
+	return hist_field;
+}
+
 struct hist_elt_data {
 	char *comm;
+	u64 *var_ref_vals;
 };
 
+static u64 hist_field_var_ref(struct hist_field *hist_field,
+			      struct tracing_map_elt *elt,
+			      struct ring_buffer_event *rbe,
+			      void *event)
+{
+	struct hist_elt_data *elt_data;
+	u64 var_val = 0;
+
+	elt_data = elt->private_data;
+	var_val = elt_data->var_ref_vals[hist_field->var_ref_idx];
+
+	return var_val;
+}
+
+static bool resolve_var_refs(struct hist_trigger_data *hist_data, void *key,
+			     u64 *var_ref_vals, bool self)
+{
+	struct hist_trigger_data *var_data;
+	struct tracing_map_elt *var_elt;
+	struct hist_field *hist_field;
+	unsigned int i, var_idx;
+	bool resolved = true;
+	u64 var_val = 0;
+
+	for (i = 0; i < hist_data->n_var_refs; i++) {
+		hist_field = hist_data->var_refs[i];
+		var_idx = hist_field->var.idx;
+		var_data = hist_field->var.hist_data;
+
+		if (var_data == NULL) {
+			resolved = false;
+			break;
+		}
+
+		if ((self && var_data != hist_data) ||
+		    (!self && var_data == hist_data))
+			continue;
+
+		var_elt = tracing_map_lookup(var_data->map, key);
+		if (!var_elt) {
+			resolved = false;
+			break;
+		}
+
+		if (!tracing_map_var_set(var_elt, var_idx)) {
+			resolved = false;
+			break;
+		}
+
+		if (self || !hist_field->read_once)
+			var_val = tracing_map_read_var(var_elt, var_idx);
+		else
+			var_val = tracing_map_read_var_once(var_elt, var_idx);
+
+		var_ref_vals[i] = var_val;
+	}
+
+	return resolved;
+}
+
 static const char *hist_field_name(struct hist_field *field,
 				   unsigned int level)
 {
@@ -331,8 +674,20 @@ static const char *hist_field_name(struct hist_field *field,
 		field_name = hist_field_name(field->operands[0], ++level);
 	else if (field->flags & HIST_FIELD_FL_TIMESTAMP)
 		field_name = "common_timestamp";
-	else if (field->flags & HIST_FIELD_FL_EXPR)
-		field_name = field->name;
+	else if (field->flags & HIST_FIELD_FL_EXPR ||
+		 field->flags & HIST_FIELD_FL_VAR_REF) {
+		if (field->system) {
+			static char full_name[MAX_FILTER_STR_VAL];
+
+			strcat(full_name, field->system);
+			strcat(full_name, ".");
+			strcat(full_name, field->event_name);
+			strcat(full_name, ".");
+			strcat(full_name, field->name);
+			field_name = full_name;
+		} else
+			field_name = field->name;
+	}
 
 	if (field_name == NULL)
 		field_name = "";
@@ -612,6 +967,9 @@ static const char *get_hist_field_flags(struct hist_field *hist_field)
 
 static void expr_field_str(struct hist_field *field, char *expr)
 {
+	if (field->flags & HIST_FIELD_FL_VAR_REF)
+		strcat(expr, "$");
+
 	strcat(expr, hist_field_name(field, 0));
 
 	if (field->flags) {
@@ -742,6 +1100,11 @@ static struct hist_field *create_hist_field(struct hist_trigger_data *hist_data,
 	if (flags & HIST_FIELD_FL_EXPR)
 		goto out; /* caller will populate */
 
+	if (flags & HIST_FIELD_FL_VAR_REF) {
+		hist_field->fn = hist_field_var_ref;
+		goto out;
+	}
+
 	if (flags & HIST_FIELD_FL_HITCOUNT) {
 		hist_field->fn = hist_field_counter;
 		hist_field->size = sizeof(u64);
@@ -835,6 +1198,144 @@ static void destroy_hist_fields(struct hist_trigger_data *hist_data)
 	}
 }
 
+static int init_var_ref(struct hist_field *ref_field,
+			struct hist_field *var_field,
+			char *system, char *event_name)
+{
+	int err = 0;
+
+	ref_field->var.idx = var_field->var.idx;
+	ref_field->var.hist_data = var_field->hist_data;
+	ref_field->size = var_field->size;
+	ref_field->is_signed = var_field->is_signed;
+	ref_field->flags |= var_field->flags &
+		(HIST_FIELD_FL_TIMESTAMP | HIST_FIELD_FL_TIMESTAMP_USECS);
+
+	if (system) {
+		ref_field->system = kstrdup(system, GFP_KERNEL);
+		if (!ref_field->system)
+			return -ENOMEM;
+	}
+
+	if (event_name) {
+		ref_field->event_name = kstrdup(event_name, GFP_KERNEL);
+		if (!ref_field->event_name) {
+			err = -ENOMEM;
+			goto free;
+		}
+	}
+
+	ref_field->name = kstrdup(var_field->var.name, GFP_KERNEL);
+	if (!ref_field->name) {
+		err = -ENOMEM;
+		goto free;
+	}
+
+	ref_field->type = kstrdup(var_field->type, GFP_KERNEL);
+	if (!ref_field->type) {
+		err = -ENOMEM;
+		goto free;
+	}
+ out:
+	return err;
+ free:
+	kfree(ref_field->system);
+	kfree(ref_field->event_name);
+	kfree(ref_field->name);
+
+	goto out;
+}
+
+static struct hist_field *create_var_ref(struct hist_field *var_field,
+					 char *system, char *event_name)
+{
+	unsigned long flags = HIST_FIELD_FL_VAR_REF;
+	struct hist_field *ref_field;
+
+	ref_field = create_hist_field(var_field->hist_data, NULL, flags, NULL);
+	if (ref_field) {
+		if (init_var_ref(ref_field, var_field, system, event_name)) {
+			destroy_hist_field(ref_field, 0);
+			return NULL;
+		}
+	}
+
+	return ref_field;
+}
+
+static bool is_var_ref(char *var_name)
+{
+	if (!var_name || strlen(var_name) < 2 || var_name[0] != '$')
+		return false;
+
+	return true;
+}
+
+static char *field_name_from_var(struct hist_trigger_data *hist_data,
+				 char *var_name)
+{
+	char *name, *field;
+	unsigned int i;
+
+	for (i = 0; i < hist_data->attrs->var_defs.n_vars; i++) {
+		name = hist_data->attrs->var_defs.name[i];
+
+		if (strcmp(var_name, name) == 0) {
+			field = hist_data->attrs->var_defs.expr[i];
+			if (contains_operator(field) || is_var_ref(field))
+				continue;
+			return field;
+		}
+	}
+
+	return NULL;
+}
+
+static char *local_field_var_ref(struct hist_trigger_data *hist_data,
+				 char *system, char *event_name,
+				 char *var_name)
+{
+	struct trace_event_call *call;
+
+	if (system && event_name) {
+		call = hist_data->event_file->event_call;
+
+		if (strcmp(system, call->class->system) != 0)
+			return NULL;
+
+		if (strcmp(event_name, trace_event_name(call)) != 0)
+			return NULL;
+	}
+
+	if (!!system != !!event_name)
+		return NULL;
+
+	if (!is_var_ref(var_name))
+		return NULL;
+
+	var_name++;
+
+	return field_name_from_var(hist_data, var_name);
+}
+
+static struct hist_field *parse_var_ref(struct hist_trigger_data *hist_data,
+					char *system, char *event_name,
+					char *var_name)
+{
+	struct hist_field *var_field = NULL, *ref_field = NULL;
+
+	if (!is_var_ref(var_name))
+		return NULL;
+
+	var_name++;
+
+	var_field = find_event_var(hist_data, system, event_name, var_name);
+	if (var_field)
+		ref_field = create_var_ref(var_field, system, event_name);
+
+	return ref_field;
+}
+
 static struct ftrace_event_field *
 parse_field(struct hist_trigger_data *hist_data, struct trace_event_file *file,
 	    char *field_str, unsigned long *flags)
@@ -891,10 +1392,40 @@ static struct hist_field *parse_atom(struct hist_trigger_data *hist_data,
 				     struct trace_event_file *file, char *str,
 				     unsigned long *flags, char *var_name)
 {
+	char *s, *ref_system = NULL, *ref_event = NULL, *ref_var = str;
 	struct ftrace_event_field *field = NULL;
 	struct hist_field *hist_field = NULL;
 	int ret = 0;
 
+	s = strchr(str, '.');
+	if (s) {
+		s = strchr(++s, '.');
+		if (s) {
+			ref_system = strsep(&str, ".");
+			if (!str) {
+				ret = -EINVAL;
+				goto out;
+			}
+			ref_event = strsep(&str, ".");
+			if (!str) {
+				ret = -EINVAL;
+				goto out;
+			}
+			ref_var = str;
+		}
+	}
+
+	s = local_field_var_ref(hist_data, ref_system, ref_event, ref_var);
+	if (!s) {
+		hist_field = parse_var_ref(hist_data, ref_system, ref_event, ref_var);
+		if (hist_field) {
+			hist_data->var_refs[hist_data->n_var_refs] = hist_field;
+			hist_field->var_ref_idx = hist_data->n_var_refs++;
+			return hist_field;
+		}
+	} else
+		str = s;
+
 	field = parse_field(hist_data, file, str, flags);
 	if (IS_ERR(field)) {
 		ret = PTR_ERR(field);
@@ -1066,6 +1597,9 @@ static struct hist_field *parse_expr(struct hist_trigger_data *hist_data,
 		goto free;
 	}
 
+	operand1->read_once = true;
+	operand2->read_once = true;
+
 	expr->operands[0] = operand1;
 	expr->operands[1] = operand2;
 	expr->operator = field_op;
@@ -1238,6 +1772,12 @@ static int create_key_field(struct hist_trigger_data *hist_data,
 			goto out;
 		}
 
+		if (hist_field->flags & HIST_FIELD_FL_VAR_REF) {
+			destroy_hist_field(hist_field, 0);
+			ret = -EINVAL;
+			goto out;
+		}
+
 		key_size = hist_field->size;
 	}
 
@@ -1576,6 +2116,7 @@ static int create_tracing_map_fields(struct hist_trigger_data *hist_data)
 
 	hist_data->attrs = attrs;
 	hist_data->remove = remove;
+	hist_data->event_file = file;
 
 	ret = create_hist_fields(hist_data, file);
 	if (ret)
@@ -1598,12 +2139,6 @@ static int create_tracing_map_fields(struct hist_trigger_data *hist_data)
 	ret = create_tracing_map_fields(hist_data);
 	if (ret)
 		goto free;
-
-	ret = tracing_map_init(hist_data->map);
-	if (ret)
-		goto free;
-
-	hist_data->event_file = file;
  out:
 	return hist_data;
  free:
@@ -1618,12 +2153,17 @@ static int create_tracing_map_fields(struct hist_trigger_data *hist_data)
 
 static void hist_trigger_elt_update(struct hist_trigger_data *hist_data,
 				    struct tracing_map_elt *elt, void *rec,
-				    struct ring_buffer_event *rbe)
+				    struct ring_buffer_event *rbe,
+				    u64 *var_ref_vals)
 {
+	struct hist_elt_data *elt_data;
 	struct hist_field *hist_field;
 	unsigned int i, var_idx;
 	u64 hist_val;
 
+	elt_data = elt->private_data;
+	elt_data->var_ref_vals = var_ref_vals;
+
 	for_each_hist_val_field(i, hist_data) {
 		hist_field = hist_data->fields[i];
 		hist_val = hist_field->fn(hist_field, elt, rbe, rec);
@@ -1675,6 +2215,7 @@ static void event_hist_trigger(struct event_trigger_data *data, void *rec,
 	struct hist_trigger_data *hist_data = data->private_data;
 	bool use_compound_key = (hist_data->n_keys > 1);
 	unsigned long entries[HIST_STACKTRACE_DEPTH];
+	u64 var_ref_vals[TRACING_MAP_VARS_MAX];
 	char compound_key[HIST_KEY_SIZE_MAX];
 	struct tracing_map_elt *elt = NULL;
 	struct stack_trace stacktrace;
@@ -1714,9 +2255,15 @@ static void event_hist_trigger(struct event_trigger_data *data, void *rec,
 	if (use_compound_key)
 		key = compound_key;
 
+	if (hist_data->n_var_refs &&
+	    !resolve_var_refs(hist_data, key, var_ref_vals, false))
+		return;
+
 	elt = tracing_map_insert(hist_data->map, key);
-	if (elt)
-		hist_trigger_elt_update(hist_data, elt, rec, rbe);
+	if (!elt)
+		return;
+
+	hist_trigger_elt_update(hist_data, elt, rec, rbe, var_ref_vals);
 }
 
 static void hist_trigger_stacktrace_print(struct seq_file *m,
@@ -1931,8 +2478,11 @@ static void hist_field_print(struct seq_file *m, struct hist_field *hist_field)
 
 	if (hist_field->flags & HIST_FIELD_FL_TIMESTAMP)
 		seq_puts(m, "common_timestamp");
-	else if (field_name)
+	else if (field_name) {
+		if (hist_field->flags & HIST_FIELD_FL_VAR_REF)
+			seq_putc(m, '$');
 		seq_printf(m, "%s", field_name);
+	}
 
 	if (hist_field->flags) {
 		const char *flags_str = get_hist_field_flags(hist_field);
@@ -2072,7 +2622,11 @@ static void event_hist_trigger_free(struct event_trigger_ops *ops,
 	if (!data->ref) {
 		if (data->name)
 			del_named_trigger(data);
+
 		trigger_data_free(data);
+
+		remove_hist_vars(hist_data);
+
 		destroy_hist_data(hist_data);
 	}
 }
@@ -2285,23 +2839,55 @@ static int hist_register_trigger(char *glob, struct event_trigger_ops *ops,
 			goto out;
 	}
 
-	list_add_rcu(&data->list, &file->triggers);
 	ret++;
 
-	update_cond_flag(file);
-
 	if (hist_data->enable_timestamps)
 		tracing_set_time_stamp_abs(file->tr, true);
+ out:
+	return ret;
+}
+
+static int hist_trigger_enable(struct event_trigger_data *data,
+			       struct trace_event_file *file)
+{
+	int ret = 0;
+
+	list_add_tail_rcu(&data->list, &file->triggers);
+
+	update_cond_flag(file);
 
 	if (trace_event_trigger_enable_disable(file, 1) < 0) {
 		list_del_rcu(&data->list);
 		update_cond_flag(file);
 		ret--;
 	}
- out:
+
 	return ret;
 }
 
+static bool hist_trigger_check_refs(struct event_trigger_data *data,
+				    struct trace_event_file *file)
+{
+	struct hist_trigger_data *hist_data = data->private_data;
+	struct event_trigger_data *test, *named_data = NULL;
+
+	if (hist_data->attrs->name)
+		named_data = find_named_trigger(hist_data->attrs->name);
+
+	list_for_each_entry_rcu(test, &file->triggers, list) {
+		if (test->cmd_ops->trigger_type == ETT_EVENT_HIST) {
+			if (!hist_trigger_match(data, test, named_data, false))
+				continue;
+			hist_data = test->private_data;
+			if (check_var_refs(hist_data))
+				return true;
+			break;
+		}
+	}
+
+	return false;
+}
+
 static void hist_unregister_trigger(char *glob, struct event_trigger_ops *ops,
 				    struct event_trigger_data *data,
 				    struct trace_event_file *file)
@@ -2334,11 +2920,30 @@ static void hist_unregister_trigger(char *glob, struct event_trigger_ops *ops,
 	}
 }
 
+static bool hist_file_check_refs(struct trace_event_file *file)
+{
+	struct hist_trigger_data *hist_data;
+	struct event_trigger_data *test;
+
+	list_for_each_entry_rcu(test, &file->triggers, list) {
+		if (test->cmd_ops->trigger_type == ETT_EVENT_HIST) {
+			hist_data = test->private_data;
+			if (check_var_refs(hist_data))
+				return true;
+		}
+	}
+
+	return false;
+}
+
 static void hist_unreg_all(struct trace_event_file *file)
 {
 	struct event_trigger_data *test, *n;
 	struct hist_trigger_data *hist_data;
 
+	if (hist_file_check_refs(file))
+		return;
+
 	list_for_each_entry_safe(test, n, &file->triggers, list) {
 		if (test->cmd_ops->trigger_type == ETT_EVENT_HIST) {
 			hist_data = test->private_data;
@@ -2414,6 +3019,11 @@ static int event_hist_trigger_func(struct event_command *cmd_ops,
 	}
 
 	if (remove) {
+		if (hist_trigger_check_refs(trigger_data, file)) {
+			ret = -EBUSY;
+			goto out_free;
+		}
+
 		cmd_ops->unreg(glob+1, trigger_ops, trigger_data, file);
 		ret = 0;
 		goto out_free;
@@ -2431,14 +3041,33 @@ static int event_hist_trigger_func(struct event_command *cmd_ops,
 		goto out_free;
 	} else if (ret < 0)
 		goto out_free;
+
+	if (get_named_trigger_data(trigger_data))
+		goto enable;
+
+	if (has_hist_vars(hist_data))
+		save_hist_vars(hist_data);
+
+	ret = tracing_map_init(hist_data->map);
+	if (ret)
+		goto out_unreg;
+enable:
+	ret = hist_trigger_enable(trigger_data, file);
+	if (ret)
+		goto out_unreg;
+
 	/* Just return zero, not the number of registered triggers */
 	ret = 0;
  out:
 	return ret;
+ out_unreg:
+	cmd_ops->unreg(glob+1, trigger_ops, trigger_data, file);
  out_free:
 	if (cmd_ops->set_filter)
 		cmd_ops->set_filter(NULL, trigger_data, NULL);
 
+	remove_hist_vars(hist_data);
+
 	kfree(trigger_data);
 
 	destroy_hist_data(hist_data);
diff --git a/kernel/trace/trace_events_trigger.c b/kernel/trace/trace_events_trigger.c
index 9b0fe31..a7a5bed 100644
--- a/kernel/trace/trace_events_trigger.c
+++ b/kernel/trace/trace_events_trigger.c
@@ -909,6 +909,12 @@ void set_named_trigger_data(struct event_trigger_data *data,
 	data->named_data = named_data;
 }
 
+struct event_trigger_data *
+get_named_trigger_data(struct event_trigger_data *data)
+{
+	return data->named_data;
+}
+
 static void
 traceon_trigger(struct event_trigger_data *data, void *rec,
 		struct ring_buffer_event *event)
-- 
1.9.3