rv: Retry when da monitor detects race conditions

DA monitor can be accessed from multiple cores simultaneously, this is
likely, for instance when dealing with per-task monitors reacting on
events that do not always occur on the CPU where the task is running.
This can cause race conditions where two events change the next state
and we see inconsistent values. E.g.:

[62] event_srs: 27: sleepable x sched_wakeup -> running (final)
[63] event_srs: 27: sleepable x sched_set_state_sleepable -> sleepable
[63] error_srs: 27: event sched_switch_suspend not expected in the state running

In this case the monitor fails because the event on CPU 62 wins against
the one on CPU 63, although the correct state should have been
sleepable, since the task get suspended.

Detect if the current state was modified by using try_cmpxchg while
storing the next value. If it was, try again reading the current state.
After a maximum number of failed retries, react by calling a special
tracepoint, print on the console and reset the monitor.

Remove the functions da_monitor_curr_state() and da_monitor_set_state()
as they only hide the underlying implementation in this case.

Monitors where this type of condition can occur must be able to account
for racing events in any possible order, as we cannot know the winner.

Cc: Ingo Molnar <[e-mail]>
Cc: Masami Hiramatsu <[e-mail]>
Cc: Tomas Glozar <[e-mail]>
Cc: Juri Lelli <[e-mail]>
Cc: Clark Williams <[e-mail]>
Cc: John Kacur <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: https://lore.kernel.org/[e-mail]
Signed-off-by: Gabriele Monaco <[e-mail]>
Reviewed-by: Nam Cao <[e-mail]>
Signed-off-by: Steven Rostedt (Google) <[e-mail]>

include/linux/rv.h

···
 #include <linux/types.h>
 #include <linux/list.h>
 
-#define MAX_DA_NAME_LEN	32
+#define MAX_DA_NAME_LEN			32
+#define MAX_DA_RETRY_RACING_EVENTS	3
 
 #ifdef CONFIG_RV
 #include <linux/bitops.h>

include/rv/da_monitor.h

···
 }												\
 												\
 /*												\
- * da_monitor_curr_state_##name - return the current state					\
- */												\
-static inline type da_monitor_curr_state_##name(struct da_monitor *da_mon)			\
-{												\
-	return da_mon->curr_state;								\
-}												\
-												\
-/*												\
- * da_monitor_set_state_##name - set the new current state					\
- */												\
-static inline void										\
-da_monitor_set_state_##name(struct da_monitor *da_mon, enum states_##name state)		\
-{												\
-	da_mon->curr_state = state;								\
-}												\
-												\
-/*												\
  * da_monitor_start_##name - start monitoring							\
  *												\
  * The monitor will ignore all events until monitoring is set to true. This			\
···
  * Event handler for implicit monitors. Implicit monitor is the one which the
  * handler does not need to specify which da_monitor to manipulate. Examples
  * of implicit monitor are the per_cpu or the global ones.
+ *
+ * Retry in case there is a race between getting and setting the next state,
+ * warn and reset the monitor if it runs out of retries. The monitor should be
+ * able to handle various orders.
  */
 #define DECLARE_DA_MON_MODEL_HANDLER_IMPLICIT(name, type)					\
 												\
 static inline bool										\
 da_event_##name(struct da_monitor *da_mon, enum events_##name event)				\
 {												\
-	type curr_state = da_monitor_curr_state_##name(da_mon);					\
-	type next_state = model_get_next_state_##name(curr_state, event);			\
+	enum states_##name curr_state, next_state;						\
 												\
-	if (next_state != INVALID_STATE) {							\
-		da_monitor_set_state_##name(da_mon, next_state);				\
-												\
-		trace_event_##name(model_get_state_name_##name(curr_state),			\
-				   model_get_event_name_##name(event),				\
-				   model_get_state_name_##name(next_state),			\
-				   model_is_final_state_##name(next_state));			\
-												\
-		return true;									\
+	curr_state = READ_ONCE(da_mon->curr_state);						\
+	for (int i = 0; i < MAX_DA_RETRY_RACING_EVENTS; i++) {					\
+		next_state = model_get_next_state_##name(curr_state, event);			\
+		if (next_state == INVALID_STATE) {						\
+			cond_react_##name(curr_state, event);					\
+			trace_error_##name(model_get_state_name_##name(curr_state),		\
+					   model_get_event_name_##name(event));			\
+			return false;								\
+		}										\
+		if (likely(try_cmpxchg(&da_mon->curr_state, &curr_state, next_state))) {	\
+			trace_event_##name(model_get_state_name_##name(curr_state),		\
+					   model_get_event_name_##name(event),			\
+					   model_get_state_name_##name(next_state),		\
+					   model_is_final_state_##name(next_state));		\
+			return true;								\
+		}										\
 	}											\
 												\
-	cond_react_##name(curr_state, event);							\
-												\
-	trace_error_##name(model_get_state_name_##name(curr_state),				\
-			   model_get_event_name_##name(event));					\
-												\
+	trace_rv_retries_error(#name, model_get_event_name_##name(event));			\
+	pr_warn("rv: " __stringify(MAX_DA_RETRY_RACING_EVENTS)					\
+		" retries reached for event %s, resetting monitor %s",				\
+		model_get_event_name_##name(event), #name);					\
 	return false;										\
 }												\
 
 /*
  * Event handler for per_task monitors.
+ *
+ * Retry in case there is a race between getting and setting the next state,
+ * warn and reset the monitor if it runs out of retries. The monitor should be
+ * able to handle various orders.
  */
 #define DECLARE_DA_MON_MODEL_HANDLER_PER_TASK(name, type)					\
 												\
 static inline bool da_event_##name(struct da_monitor *da_mon, struct task_struct *tsk,		\
 				   enum events_##name event)					\
 {												\
-	type curr_state = da_monitor_curr_state_##name(da_mon);					\
-	type next_state = model_get_next_state_##name(curr_state, event);			\
+	enum states_##name curr_state, next_state;						\
 												\
-	if (next_state != INVALID_STATE) {							\
-		da_monitor_set_state_##name(da_mon, next_state);				\
-												\
-		trace_event_##name(tsk->pid,							\
-				   model_get_state_name_##name(curr_state),			\
-				   model_get_event_name_##name(event),				\
-				   model_get_state_name_##name(next_state),			\
-				   model_is_final_state_##name(next_state));			\
-												\
-		return true;									\
+	curr_state = READ_ONCE(da_mon->curr_state);						\
+	for (int i = 0; i < MAX_DA_RETRY_RACING_EVENTS; i++) {					\
+		next_state = model_get_next_state_##name(curr_state, event);			\
+		if (next_state == INVALID_STATE) {						\
+			cond_react_##name(curr_state, event);					\
+			trace_error_##name(tsk->pid,						\
+					   model_get_state_name_##name(curr_state),		\
+					   model_get_event_name_##name(event));			\
+			return false;								\
+		}										\
+		if (likely(try_cmpxchg(&da_mon->curr_state, &curr_state, next_state))) {	\
+			trace_event_##name(tsk->pid,						\
+					   model_get_state_name_##name(curr_state),		\
+					   model_get_event_name_##name(event),			\
+					   model_get_state_name_##name(next_state),		\
+					   model_is_final_state_##name(next_state));		\
+			return true;								\
+		}										\
 	}											\
 												\
-	cond_react_##name(curr_state, event);							\
-												\
-	trace_error_##name(tsk->pid,								\
-			   model_get_state_name_##name(curr_state),				\
-			   model_get_event_name_##name(event));					\
-												\
+	trace_rv_retries_error(#name, model_get_event_name_##name(event));			\
+	pr_warn("rv: " __stringify(MAX_DA_RETRY_RACING_EVENTS)					\
+		" retries reached for event %s, resetting monitor %s",				\
+		model_get_event_name_##name(event), #name);					\
 	return false;										\
 }
 

kernel/trace/rv/Kconfig

···
 config RV_MON_EVENTS
 	bool
 
+config RV_MON_MAINTENANCE_EVENTS
+	bool
+
 config DA_MON_EVENTS_IMPLICIT
 	select RV_MON_EVENTS
+	select RV_MON_MAINTENANCE_EVENTS
 	bool
 
 config DA_MON_EVENTS_ID
 	select RV_MON_EVENTS
+	select RV_MON_MAINTENANCE_EVENTS
 	bool
 
 config LTL_MON_EVENTS_ID

kernel/trace/rv/rv_trace.h

···
 #include <monitors/sleep/sleep_trace.h>
 // Add new monitors based on CONFIG_LTL_MON_EVENTS_ID here
 #endif /* CONFIG_LTL_MON_EVENTS_ID */
+
+#ifdef CONFIG_RV_MON_MAINTENANCE_EVENTS
+/* Tracepoint useful for monitors development, currenly only used in DA */
+TRACE_EVENT(rv_retries_error,
+
+	TP_PROTO(char *name, char *event),
+
+	TP_ARGS(name, event),
+
+	TP_STRUCT__entry(
+		__string(	name,	name	)
+		__string(	event,	event	)
+	),
+
+	TP_fast_assign(
+		__assign_str(name);
+		__assign_str(event);
+	),
+
+	TP_printk(__stringify(MAX_DA_RETRY_RACING_EVENTS)
+		" retries reached for event %s, resetting monitor %s",
+		__get_str(event), __get_str(name))
+);
+#endif /* CONFIG_RV_MON_MAINTENANCE_EVENTS */
 #endif /* _TRACE_RV_H */
 
 /* This part must be outside protection */