linux-kernel - Re: [PATCH RFC context_tracking] Make RCU watch ct_kernel_exit

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Message-ID: <xhsmhjza4k5e5.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Wed, 05 Feb 2025 12:17:06 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: paulmck@...nel.org, linux-kernel@...r.kernel.org
Cc: frederic@...nel.org, leitao@...ian.org
Subject: Re: [PATCH RFC context_tracking] Make RCU watch
 ct_kernel_exit_state() warning

On 01/02/25 10:44, Paul E. McKenney wrote:
> The WARN_ON_ONCE() in ct_kernel_exit_state() follows the call to
> ct_state_inc(), which means that RCU is not watching this WARN_ON_ONCE().
> This can (and does) result in extraneous lockdep warnings when this
> WARN_ON_ONCE() triggers.  These extraneous warnings are the opposite
> of helpful.
>
> Therefore, invert the WARN_ON_ONCE() condition and move it before the
> call to ct_state_inc().  This does mean that the ct_state_inc() return
> value can no longer be used in the WARN_ON_ONCE() condition, so discard
> this return value and instead use a call to rcu_is_watching_curr_cpu().
> This call is executed only in CONFIG_RCU_EQS_DEBUG=y kernels, so there
> is no added overhead in production use.
>
> Reported-by: Breno Leitao <leitao@...ian.org>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> Cc: Frederic Weisbecker <frederic@...nel.org>
> Cc: Valentin Schneider <vschneid@...hat.com>
>
> diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
> index 938c48952d26..fb5be6e9b423 100644
> --- a/kernel/context_tracking.c
> +++ b/kernel/context_tracking.c
> @@ -80,17 +80,16 @@ static __always_inline void rcu_task_trace_heavyweight_exit(void)
>   */
>  static noinstr void ct_kernel_exit_state(int offset)
>  {
> -	int seq;
> -
>       /*
>        * CPUs seeing atomic_add_return() must see prior RCU read-side
>        * critical sections, and we also must force ordering with the
>        * next idle sojourn.
>        */
>       rcu_task_trace_heavyweight_enter();  // Before CT state update!
> -	seq = ct_state_inc(offset);
> -	// RCU is no longer watching.  Better be in extended quiescent state!
> -	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && (seq & CT_RCU_WATCHING));
> +	// RCU is still watching.  Better not be in extended quiescent state!
> +	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !rcu_is_watching_curr_cpu());

Isn't this equivalent to the check done in ct_kernel_enter_state()? That
is, it operates on the same context_tracking.state value that the
ct_kernel_enter_state() WARN_ON_ONCE() sees, so if the warning is to fire
it will fire there first.

I don't have any better idea than something like the ugly:

	if (IS_ENABLED(CONFIG_RCU_EQS_DEBUG)) {
		unsigned int new_state, state = atomic_read(&ct->state);
		bool ret;

		do {
			new_state = state + offset;
			// RCU will no longer be watching. Better be in extended quiescent state!
			WARN_ON_ONCE(new_state & CT_RCU_WATCHING);

			ret = atomic_try_cmpxchg(&ct->state, &state, new_state);
		} while (!ret);
	} else {
		(void)ct_state_inc(offset);
	}

> +	(void)ct_state_inc(offset);
> +	// RCU is no longer watching.
>  }
>
>  /*