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

[Frederic Weisbecker <frederic@kernel.org>]

On Wed, Mar 03, 2021 at 04:26:30PM -0800, paulmck@kernel.org wrote:
>  /**
> + * start_poll_state_synchronize_rcu - Snapshot and start RCU grace period
> + *
> + * Returns a cookie that is used by a later call to cond_synchronize_rcu()
> + * or poll_state_synchronize_rcu() to determine whether or not a full
> + * grace period has elapsed in the meantime.  If the needed grace period
> + * is not already slated to start, notifies RCU core of the need for that
> + * grace period.
> + *
> + * Interrupts must be enabled for the case where it is necessary to awaken
> + * the grace-period kthread.
> + */
> +unsigned long start_poll_synchronize_rcu(void)
> +{
> +	unsigned long flags;
> +	unsigned long gp_seq = get_state_synchronize_rcu();

Ah! It's using rcu_seq_snap() and not rcu_seq_current() and therefore it's
waiting for a safe future grace period, right?

If so, please discard my previous email.

Thanks.