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      /*
 * Core SRCU state machine.  Advance callbacks from ->batch_check0 to
 * ->batch_check1 and then to ->batch_done as readers drain.
 */
static void srcu_advance_batches(struct srcu_struct *sp, int trycount)
{
	int idx = 1 ^ (sp->completed & 1);

	/*
	 * Because readers might be delayed for an extended period after
	 * fetching ->completed for their index, at any point in time there
	 * might well be readers using both idx=0 and idx=1.  We therefore
	 * need to wait for readers to clear from both index values before
	 * invoking a callback.
	 */

	if (rcu_batch_empty(&sp->batch_check0) &&
	    rcu_batch_empty(&sp->batch_check1))
		return; /* no callbacks need to be advanced */

	if (!try_check_zero(sp, idx, trycount))
		return; /* failed to advance, will try after SRCU_INTERVAL */

	/*
	 * The callbacks in ->batch_check1 have already done with their
	 * first zero check and flip back when they were enqueued on
	 * ->batch_check0 in a previous invocation of srcu_advance_batches().
	 * (Presumably try_check_zero() returned false during that
	 * invocation, leaving the callbacks stranded on ->batch_check1.)
	 * They are therefore ready to invoke, so move them to ->batch_done.
	 */
	rcu_batch_move(&sp->batch_done, &sp->batch_check1);

	if (rcu_batch_empty(&sp->batch_check0))
		return; /* no callbacks need to be advanced */
	srcu_flip(sp);

	/*
	 * The callbacks in ->batch_check0 just finished their
	 * first check zero and flip, so move them to ->batch_check1
	 * for future checking on the other idx.
	 */
	rcu_batch_move(&sp->batch_check1, &sp->batch_check0);

	/*
	 * SRCU read-side critical sections are normally short, so check
	 * at least twice in quick succession after a flip.
	 */
	trycount = trycount < 2 ? 2 : trycount;
	if (!try_check_zero(sp, idx^1, trycount))
		return; /* failed to advance, will try after SRCU_INTERVAL */

	/*
	 * The callbacks in ->batch_check1 have now waited for all
	 * pre-existing readers using both idx values.  They are therefore
	 * ready to invoke, so move them to ->batch_done.
	 */
	rcu_batch_move(&sp->batch_done, &sp->batch_check1);
}

/*
 * Invoke a limited number of SRCU callbacks that have passed through
 * their grace period.  If there are more to do, SRCU will reschedule
 * the workqueue.
 */
static void srcu_invoke_callbacks(struct srcu_struct *sp)
{
	int i;
	struct rcu_head *head;

	for (i = 0; i < SRCU_CALLBACK_BATCH; i++) {
		head = rcu_batch_dequeue(&sp->batch_done);
		if (!head)
			break;
		local_bh_disable();
		head->func(head);
		local_bh_enable();
	}
}

/*
 * Finished one round of SRCU grace period.  Start another if there are
 * more SRCU callbacks queued, otherwise put SRCU into not-running state.
 */
static void srcu_reschedule(struct srcu_struct *sp)
{
	bool pending = true;

	if (rcu_batch_empty(&sp->batch_done) &&
	    rcu_batch_empty(&sp->batch_check1) &&
	    rcu_batch_empty(&sp->batch_check0) &&
	    rcu_batch_empty(&sp->batch_queue)) {
		spin_lock_irq(&sp->queue_lock);
		if (rcu_batch_empty(&sp->batch_done) &&
		    rcu_batch_empty(&sp->batch_check1) &&
		    rcu_batch_empty(&sp->batch_check0) &&
		    rcu_batch_empty(&sp->batch_queue)) {
			sp->running = false;
			pending = false;
		}
		spin_unlock_irq(&sp->queue_lock);
	}

	if (pending)
		queue_delayed_work(system_power_efficient_wq,
				   &sp->work, SRCU_INTERVAL);
}

/*
 * This is the work-queue function that handles SRCU grace periods.
 */
void process_srcu(struct work_struct *work)
{
	struct srcu_struct *sp;

	sp = container_of(work, struct srcu_struct, work.work);

	srcu_collect_new(sp);
	srcu_advance_batches(sp, 1);
	srcu_invoke_callbacks(sp);
	srcu_reschedule(sp);
}
EXPORT_SYMBOL_GPL(process_srcu);