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    SubjectRe: [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@kernel.org wrote:
    > From: "Paul E. McKenney" <paulmck@kernel.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@kernel.org>

    Thanks!

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