linux-kernel - Re: [PATCH tip/core/rcu 1/3] rcu: Provide polling interfaces for Tree RCU grace periods

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Date:   Tue, 16 Mar 2021 15:47:22 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     paulmck@...nel.org
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org
Subject: Re: [PATCH tip/core/rcu 1/3] rcu: Provide polling interfaces for
 Tree RCU grace periods

On Wed, Mar 03, 2021 at 04:26:30PM -0800, paulmck@...nel.org wrote:
> +/**
> + * poll_state_synchronize_rcu - Conditionally wait for an RCU grace period
> + *
> + * @oldstate: return from call to get_state_synchronize_rcu() or start_poll_synchronize_rcu()
> + *
> + * If a full RCU grace period has elapsed since the earlier call from
> + * which oldstate was obtained, return @true, otherwise return @false.
> + * Otherwise, invoke synchronize_rcu() to wait for a full grace period.
> + *
> + * Yes, this function does not take counter wrap into account.
> + * But counter wrap is harmless.  If the counter wraps, we have waited for
> + * more than 2 billion grace periods (and way more on a 64-bit system!).
> + * Those needing to keep oldstate values for very long time periods
> + * (many hours even on 32-bit systems) should check them occasionally
> + * and either refresh them or set a flag indicating that the grace period
> + * has completed.
> + */
> +bool poll_state_synchronize_rcu(unsigned long oldstate)
> +{
> +	if (rcu_seq_done(&rcu_state.gp_seq, oldstate)) {
> +		smp_mb(); /* Ensure GP ends before subsequent accesses. */
> +		return true;
> +	}
> +	return false;
> +}
> +EXPORT_SYMBOL_GPL(poll_state_synchronize_rcu);
> +
> +/**
>   * cond_synchronize_rcu - Conditionally wait for an RCU grace period
>   *
>   * @oldstate: return value from earlier call to get_state_synchronize_rcu()
>   *
>   * If a full RCU grace period has elapsed since the earlier call to
> - * get_state_synchronize_rcu(), just return.  Otherwise, invoke
> - * synchronize_rcu() to wait for a full grace period.
> + * get_state_synchronize_rcu() or start_poll_synchronize_rcu(), just return.
> + * Otherwise, invoke synchronize_rcu() to wait for a full grace period.
>   *
>   * Yes, this function does not take counter wrap into account.  But
>   * counter wrap is harmless.  If the counter wraps, we have waited for
> @@ -3804,7 +3864,7 @@ EXPORT_SYMBOL_GPL(get_state_synchronize_rcu);
>   */
>  void cond_synchronize_rcu(unsigned long oldstate)
>  {
> -	if (!rcu_seq_done(&rcu_state.gp_seq, oldstate))
> +	if (!poll_state_synchronize_rcu(oldstate))
>  		synchronize_rcu();
>  	else
>  		smp_mb(); /* Ensure GP ends before subsequent accesses. */

poll_state_synchronize_rcu() already does the full barrier.

> -- 
> 2.9.5
>