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

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Date:   Fri, 19 Mar 2021 14:58:54 +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:
> From: "Paul E. McKenney" <paulmck@...nel.org>
> 
> There is a need for a non-blocking polling interface for RCU grace
> periods, so this commit supplies start_poll_synchronize_rcu() and
> poll_state_synchronize_rcu() for this purpose.  Note that the existing
> get_state_synchronize_rcu() may be used if future grace periods are
> inevitable (perhaps due to a later call_rcu() invocation).  The new
> start_poll_synchronize_rcu() is to be used if future grace periods
> might not otherwise happen.  Finally, poll_state_synchronize_rcu()
> provides a lockless check for a grace period having elapsed since
> the corresponding call to either of the get_state_synchronize_rcu()
> or start_poll_synchronize_rcu().
> 
> As with get_state_synchronize_rcu(), the return value from either
> get_state_synchronize_rcu() or start_poll_synchronize_rcu() is passed in
> to a later call to either poll_state_synchronize_rcu() or the existing
> (might_sleep) cond_synchronize_rcu().

It's all a matter of personal taste but if I may suggest some namespace
modifications:

get_state_synchronize_rcu() -> synchronize_rcu_poll_start_raw()
start_poll_synchronize_rcu() -> synchronize_rcu_poll_start()
poll_state_synchronize_rcu() -> synchronize_rcu_poll()
cond_synchronize_rcu() -> synchronize_rcu_cond()

But it's up to you really.

>  /**
> + * 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()

It may be worth noting that calling start_poll_synchronize_rcu() and then
pass the cookie to cond_synchronize_rcu() soon after may end up waiting for
one more grace period.

> + * 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();
> +	bool needwake;
> +	struct rcu_data *rdp;
> +	struct rcu_node *rnp;
[...]
> +
> +/**
> + * 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.

Rephrase suggestion for the last sentence:

"In case of failure, it's up to the caller to try polling again later or
invoke synchronize_rcu() to wait for a new full grace period to complete."


In any case: Reviewed-by: Frederic Weisbecker <frederic@...nel.org>

Thanks!