linux-kernel - Re: [RFC PATCH v2 10/12] rv: Retry when da monitor detects race conditions

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Message-ID: <20250519090626.zjiYgUGW@linutronix.de>
Date: Mon, 19 May 2025 11:06:26 +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>
Subject: Re: [RFC PATCH v2 10/12] rv: Retry when da monitor detects race
 conditions

On Wed, May 14, 2025 at 10:43:12AM +0200, Gabriele Monaco wrote:
> -static inline void										\
> -da_monitor_set_state_##name(struct da_monitor *da_mon, enum states_##name state)		\
> +static inline bool										\
> +da_monitor_set_state_##name(struct da_monitor *da_mon, enum states_##name prev_state,		\
> +			    enum states_##name state)						\
>  {												\
> -	da_mon->curr_state = state;								\
> +	return try_cmpxchg(&da_mon->curr_state, &prev_state, state);				\
>  }												\

This is a very thin wrapper. Should we just call try_cmpxchg() directly?

>  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);			\
> +	bool changed;										\
> +	type curr_state, next_state;								\
>  												\
> -	if (next_state != INVALID_STATE) {							\
> -		da_monitor_set_state_##name(da_mon, next_state);				\
> +	for (int i = 0; i < MAX_DA_RETRY_RACING_EVENTS; i++) {					\
> +		curr_state = da_monitor_curr_state_##name(da_mon);				\

For the follow-up iterations (i > 0), it is not necessary to read
curr_state again here, we already have its value from try_cmpxchg() below.

Also, thinking about memory barrier hurts my main, but I'm not entirely
sure if reading curr_state without memory barrier here is okay.

How about something like below?

curr_state = da_monitor_curr_state_##name(da_mon);
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)
		break;
	if (try_cmpxchg(&da_mon->curr_state, &curr_state, next_state))
		break;
}

Furthermore, it is possible to replace for(...) with while (1)? I don't
think we can have a live lock, because if we fail to do try_cmpxchg(),
the "other guy" surely succeed.

Best regards,
Nam