diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index f743fe19..4045e105 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -7744,16 +7744,17 @@ static int instance_mkdir(const char *name)
 	raw_spin_lock_init(&tr->start_lock);
 
 	tr->max_lock = (arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED;
 
 	tr->current_trace = &nop_trace;
 
 	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;
 
 	tr->dir = tracefs_create_dir(name, trace_instance_dir);
 	if (!tr->dir)
 		goto out_free_tr;
 
@@ -8401,16 +8402,17 @@ __init static int tracer_alloc_buffers(void)
 				       &trace_panic_notifier);
 
 	register_die_notifier(&trace_die_notifier);
 
 	global_trace.flags = TRACE_ARRAY_FL_GLOBAL;
 
 	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();
 
 	register_snapshot_cmd();
 
 	return 0;
 
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index a94cc69b..630a341f 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -269,16 +269,17 @@ struct trace_array {
 	struct list_head	func_probes;
 	struct list_head	mod_trace;
 	struct list_head	mod_notrace;
 #endif
 	/* function tracing enabled */
 	int			function_enabled;
 #endif
 	int			time_stamp_abs_ref;
+	struct list_head	hist_vars;
 };
 
 enum {
 	TRACE_ARRAY_FL_GLOBAL	= (1 << 0)
 };
 
 extern struct list_head ftrace_trace_arrays;
 
@@ -1545,16 +1546,18 @@ extern struct event_trigger_data *find_named_trigger(const char *name);
 extern bool is_named_trigger(struct event_trigger_data *test);
 extern int save_named_trigger(const char *name,
 			      struct event_trigger_data *data);
 extern void del_named_trigger(struct event_trigger_data *data);
 extern void pause_named_trigger(struct event_trigger_data *data);
 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);
 
 /**
  * struct event_trigger_ops - callbacks for trace event triggers
  *
  * The methods in this structure provide per-event trigger hooks for
diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index e30bd86b..dbcdd2ff 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -54,17 +54,22 @@ struct hist_field {
 	unsigned int			size;
 	unsigned int			offset;
 	unsigned int                    is_signed;
 	const char			*type;
 	struct hist_field		*operands[HIST_FIELD_OPERANDS_MAX];
 	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,
 			   struct tracing_map_elt *elt,
 			   struct ring_buffer_event *rbe,
 			   void *event)
 {
 	return 0;
@@ -209,16 +214,17 @@ enum hist_field_flags {
 	HIST_FIELD_FL_EXECNAME		= 1 << 6,
 	HIST_FIELD_FL_SYSCALL		= 1 << 7,
 	HIST_FIELD_FL_STACKTRACE	= 1 << 8,
 	HIST_FIELD_FL_LOG2		= 1 << 9,
 	HIST_FIELD_FL_TIMESTAMP		= 1 << 10,
 	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 {
 	unsigned int	n_vars;
 	char		*name[TRACING_MAP_VARS_MAX];
 	char		*expr[TRACING_MAP_VARS_MAX];
 };
 
@@ -248,16 +254,18 @@ struct hist_trigger_data {
 	unsigned int			key_size;
 	struct tracing_map_sort_key	sort_keys[TRACING_MAP_SORT_KEYS_MAX];
 	unsigned int			n_sort_keys;
 	struct trace_event_file		*event_file;
 	struct hist_trigger_attrs	*attrs;
 	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,
 				struct tracing_map_elt *elt,
 				struct ring_buffer_event *rbe,
 				void *event)
 {
 	struct hist_trigger_data *hist_data = hist_field->hist_data;
@@ -266,16 +274,224 @@ static u64 hist_field_timestamp(struct hist_field *hist_field,
 	u64 ts = ring_buffer_event_time_stamp(rbe);
 
 	if (hist_data->attrs->ts_in_usecs && trace_clock_in_ns(tr))
 		ts = ns2usecs(ts);
 
 	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)
 {
 	struct hist_field *hist_field, *found = NULL;
 	int i;
 
 	for_each_hist_field(i, hist_data) {
 		hist_field = hist_data->fields[i];
@@ -308,36 +524,175 @@ static struct hist_field *find_var(struct hist_trigger_data *hist_data,
 			if (hist_field)
 				return hist_field;
 		}
 	}
 
 	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)
 {
 	const char *field_name = "";
 
 	if (level > 1)
 		return field_name;
 
 	if (field->field)
 		field_name = field->field->name;
 	else if (field->flags & HIST_FIELD_FL_LOG2)
 		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 = "";
 
 	return field_name;
 }
 
 static hist_field_fn_t select_value_fn(int field_size, int field_is_signed)
@@ -607,16 +962,19 @@ static const char *get_hist_field_flags(struct hist_field *hist_field)
 	else if (hist_field->flags & HIST_FIELD_FL_TIMESTAMP_USECS)
 		flags_str = "usecs";
 
 	return flags_str;
 }
 
 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) {
 		const char *flags_str = get_hist_field_flags(field);
 
 		if (flags_str) {
 			strcat(expr, ".");
 			strcat(expr, flags_str);
@@ -737,16 +1095,21 @@ static struct hist_field *create_hist_field(struct hist_trigger_data *hist_data,
 	if (!hist_field)
 		return NULL;
 
 	hist_field->hist_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);
 		hist_field->type = kstrdup("u64", GFP_KERNEL);
 		if (!hist_field->type)
 			goto free;
 		goto out;
 	}
@@ -830,16 +1193,154 @@ static void destroy_hist_fields(struct hist_trigger_data *hist_data)
 	for (i = 0; i < HIST_FIELDS_MAX; i++) {
 		if (hist_data->fields[i]) {
 			destroy_hist_field(hist_data->fields[i], 0);
 			hist_data->fields[i] = NULL;
 		}
 	}
 }
 
+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)
 {
 	struct ftrace_event_field *field = NULL;
 	char *field_name, *modifier, *str;
 
 	modifier = str = kstrdup(field_str, GFP_KERNEL);
@@ -886,20 +1387,50 @@ parse_field(struct hist_trigger_data *hist_data, struct trace_event_file *file,
 
 	return field;
 }
 
 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);
 		goto out;
 	}
 
 	hist_field = create_hist_field(hist_data, field, *flags, var_name);
 	if (!hist_field) {
@@ -1061,16 +1592,19 @@ static struct hist_field *parse_expr(struct hist_trigger_data *hist_data,
 		(HIST_FIELD_FL_TIMESTAMP | HIST_FIELD_FL_TIMESTAMP_USECS);
 
 	expr = create_hist_field(hist_data, NULL, flags, var_name);
 	if (!expr) {
 		ret = -ENOMEM;
 		goto free;
 	}
 
+	operand1->read_once = true;
+	operand2->read_once = true;
+
 	expr->operands[0] = operand1;
 	expr->operands[1] = operand2;
 	expr->operator = field_op;
 	expr->name = expr_str(expr, 0);
 	expr->type = kstrdup(operand1->type, GFP_KERNEL);
 	if (!expr->type) {
 		ret = -ENOMEM;
 		goto free;
@@ -1233,16 +1767,22 @@ static int create_key_field(struct hist_trigger_data *hist_data,
 	} else {
 		hist_field = parse_expr(hist_data, file, field_str, flags,
 					NULL, 0);
 		if (IS_ERR(hist_field)) {
 			ret = PTR_ERR(hist_field);
 			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;
 	}
 
 	hist_data->fields[key_idx] = hist_field;
 
 	key_size = ALIGN(key_size, sizeof(u64));
 	hist_data->fields[key_idx]->size = key_size;
 	hist_data->fields[key_idx]->offset = key_offset;
@@ -1571,16 +2111,17 @@ create_hist_data(unsigned int map_bits,
 	int ret = 0;
 
 	hist_data = kzalloc(sizeof(*hist_data), GFP_KERNEL);
 	if (!hist_data)
 		return ERR_PTR(-ENOMEM);
 
 	hist_data->attrs = attrs;
 	hist_data->remove = remove;
+	hist_data->event_file = file;
 
 	ret = create_hist_fields(hist_data, file);
 	if (ret)
 		goto free;
 
 	ret = create_sort_keys(hist_data);
 	if (ret)
 		goto free;
@@ -1593,42 +2134,41 @@ create_hist_data(unsigned int map_bits,
 		ret = PTR_ERR(hist_data->map);
 		hist_data->map = NULL;
 		goto free;
 	}
 
 	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:
 	hist_data->attrs = NULL;
 
 	destroy_hist_data(hist_data);
 
 	hist_data = ERR_PTR(ret);
 
 	goto out;
 }
 
 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);
 		if (hist_field->flags & HIST_FIELD_FL_VAR) {
 			var_idx = hist_field->var.idx;
 			tracing_map_set_var(elt, var_idx, hist_val);
 			continue;
 		}
@@ -1670,16 +2210,17 @@ static inline void add_to_key(char *compound_key, void *key,
 }
 
 static void event_hist_trigger(struct event_trigger_data *data, void *rec,
 			       struct ring_buffer_event *rbe)
 {
 	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;
 	struct hist_field *key_field;
 	u64 field_contents;
 	void *key = NULL;
 	unsigned int i;
 
@@ -1709,19 +2250,25 @@ static void event_hist_trigger(struct event_trigger_data *data, void *rec,
 
 		if (use_compound_key)
 			add_to_key(compound_key, key, key_field, 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,
 					  unsigned long *stacktrace_entries,
 					  unsigned int max_entries)
 {
 	char str[KSYM_SYMBOL_LEN];
 	unsigned int spaces = 8;
@@ -1926,18 +2473,21 @@ static void hist_field_print(struct seq_file *m, struct hist_field *hist_field)
 {
 	const char *field_name = hist_field_name(hist_field, 0);
 
 	if (hist_field->var.name)
 		seq_printf(m, "%s=", hist_field->var.name);
 
 	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);
 
 		if (flags_str)
 			seq_printf(m, ".%s", flags_str);
 	}
 }
@@ -2067,17 +2617,21 @@ static void event_hist_trigger_free(struct event_trigger_ops *ops,
 
 	if (WARN_ON_ONCE(data->ref <= 0))
 		return;
 
 	data->ref--;
 	if (!data->ref) {
 		if (data->name)
 			del_named_trigger(data);
+
 		trigger_data_free(data);
+
+		remove_hist_vars(hist_data);
+
 		destroy_hist_data(hist_data);
 	}
 }
 
 static struct event_trigger_ops event_hist_trigger_ops = {
 	.func			= event_hist_trigger,
 	.print			= event_hist_trigger_print,
 	.init			= event_hist_trigger_init,
@@ -2280,33 +2834,65 @@ static int hist_register_trigger(char *glob, struct event_trigger_ops *ops,
 	}
 
 	if (data->ops->init) {
 		ret = data->ops->init(data->ops, data);
 		if (ret < 0)
 			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)
 {
 	struct hist_trigger_data *hist_data = data->private_data;
 	struct event_trigger_data *test, *named_data = NULL;
 	bool unregistered = false;
 
@@ -2329,21 +2915,40 @@ static void hist_unregister_trigger(char *glob, struct event_trigger_ops *ops,
 		test->ops->free(test->ops, test);
 
 	if (hist_data->enable_timestamps) {
 		if (!hist_data->remove || unregistered)
 			tracing_set_time_stamp_abs(file->tr, false);
 	}
 }
 
+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;
 			list_del_rcu(&test->list);
 			trace_event_trigger_enable_disable(file, 0);
 			update_cond_flag(file);
 			if (hist_data->enable_timestamps)
 				tracing_set_time_stamp_abs(file->tr, false);
@@ -2409,16 +3014,21 @@ static int event_hist_trigger_func(struct event_command *cmd_ops,
 	/* if param is non-empty, it's supposed to be a filter */
 	if (param && cmd_ops->set_filter) {
 		ret = cmd_ops->set_filter(param, trigger_data, file);
 		if (ret < 0)
 			goto out_free;
 	}
 
 	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;
 	}
 
 	ret = cmd_ops->reg(glob, trigger_ops, trigger_data, file);
 	/*
 	 * The above returns on success the # of triggers registered,
@@ -2426,24 +3036,43 @@ static int event_hist_trigger_func(struct event_command *cmd_ops,
 	 * triggers registered a failure too.
 	 */
 	if (!ret) {
 		if (!(attrs->pause || attrs->cont || attrs->clear))
 			ret = -ENOENT;
 		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);
 	goto out;
 }
 
 static struct event_command trigger_hist_cmd = {
 	.name			= "hist",
diff --git a/kernel/trace/trace_events_trigger.c b/kernel/trace/trace_events_trigger.c
index 4c269f2e..24d42350 100644
--- a/kernel/trace/trace_events_trigger.c
+++ b/kernel/trace/trace_events_trigger.c
@@ -910,16 +910,22 @@ void unpause_named_trigger(struct event_trigger_data *data)
  * given trigger.
  */
 void set_named_trigger_data(struct event_trigger_data *data,
 			    struct event_trigger_data *named_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)
 {
 	if (tracing_is_on())
 		return;
 
 	tracing_on();
