Re: [PATCH v4 10/14] rv: Retry when da monitor detects race conditions

[Nam Cao <namcao@linutronix.de>]

On Mon, Jul 21, 2025 at 10:23:20AM +0200, Gabriele Monaco wrote:
> 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 <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
> ---
>
>  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);			\
> -												\
> -	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;									\
> +	enum states_##name curr_state, next_state;						\
> +												\
> +	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, smp_processor_id());					\
> +	pr_warn("rv: " __stringify(MAX_DA_RETRY_RACING_EVENTS)					\
> +		" retries reached, resetting monitor %s", #name);				\

smp_processor_id() requires preemption to be disabled.

At the moment, trace point handler is called with preemption disabled, so
we are fine. But there is plan to change that:
https://lore.kernel.org/lkml/20241206120709.736f943e@gandalf.local.home/T/#u

Perhaps use get_cpu() and put_cpu() instead?

Nam