linux-kernel - Re: [PATCH v3 11/17] rv: Retry when da monitor detects race conditions

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Message-ID: <20250715152322.Os4lDq_B@linutronix.de>
Date: Tue, 15 Jul 2025 17:23:22 +0200
From: Nam Cao <namcao@...utronix.de>
To: Gabriele Monaco <gmonaco@...hat.com>
Cc: linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	linux-trace-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Tomas Glozar <tglozar@...hat.com>, Juri Lelli <jlelli@...hat.com>,
	Clark Williams <williams@...hat.com>,
	John Kacur <jkacur@...hat.com>
Subject: Re: [PATCH v3 11/17] rv: Retry when da monitor detects race
 conditions

On Tue, Jul 15, 2025 at 09:14:28AM +0200, Gabriele Monaco wrote:
>  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)						\
> +			goto out_react;								\
> +		if (likely(try_cmpxchg(&da_mon->curr_state, &curr_state, next_state)))		\
> +			goto out_success;							\
>  	}											\
> +	/* Special invalid transition if we run out of retries. */				\
> +	curr_state = INVALID_STATE;								\
>  												\
> +out_react:											\
>  	cond_react_##name(curr_state, event);							\
>  												\
>  	trace_error_##name(model_get_state_name_##name(curr_state),				\
>  			   model_get_event_name_##name(event));					\

If I understand correctly, if after 3 tries and we still fail to change the
state, we will invoke the reactor and trace_error? Doesn't that cause a
false positive? Because it is not a violation of the model, it is just a
race making us fail to change the state.

Same below.

Also, I wouldn't use goto unless necessary. Perhaps it is better to put the
code at "out_react:" and "out_success:" into the loop. But that's just my
personal preference, up to you.

Nam