linux/drivers/md/dm-vdo/funnel-queue.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * Copyright 2023 Red Hat
 */

#include "funnel-queue.h"

#include "cpu.h"
#include "memory-alloc.h"
#include "permassert.h"

int vdo_make_funnel_queue(struct funnel_queue **queue_ptr)
{
	int result;
	struct funnel_queue *queue;

	result = vdo_allocate(1, struct funnel_queue, "funnel queue", &queue);
	if (result != VDO_SUCCESS)
		return result;

	/*
	 * Initialize the stub entry and put it in the queue, establishing the invariant that
	 * queue->newest and queue->oldest are never null.
	 */
	queue->stub.next = NULL;
	queue->newest = &queue->stub;
	queue->oldest = &queue->stub;

	*queue_ptr = queue;
	return VDO_SUCCESS;
}

void vdo_free_funnel_queue(struct funnel_queue *queue)
{
	vdo_free(queue);
}

static struct funnel_queue_entry *get_oldest(struct funnel_queue *queue)
{
	/*
	 * Barrier requirements: We need a read barrier between reading a "next" field pointer
	 * value and reading anything it points to. There's an accompanying barrier in
	 * vdo_funnel_queue_put() between its caller setting up the entry and making it visible.
	 */
	struct funnel_queue_entry *oldest = queue->oldest;
	struct funnel_queue_entry *next = READ_ONCE(oldest->next);

	if (oldest == &queue->stub) {
		/*
		 * When the oldest entry is the stub and it has no successor, the queue is
		 * logically empty.
		 */
		if (next == NULL)
			return NULL;
		/*
		 * The stub entry has a successor, so the stub can be dequeued and ignored without
		 * breaking the queue invariants.
		 */
		oldest = next;
		queue->oldest = oldest;
		next = READ_ONCE(oldest->next);
	}

	/*
	 * We have a non-stub candidate to dequeue. If it lacks a successor, we'll need to put the
	 * stub entry back on the queue first.
	 */
	if (next == NULL) {
		struct funnel_queue_entry *newest = READ_ONCE(queue->newest);

		if (oldest != newest) {
			/*
			 * Another thread has already swung queue->newest atomically, but not yet
			 * assigned previous->next. The queue is really still empty.
			 */
			return NULL;
		}

		/*
		 * Put the stub entry back on the queue, ensuring a successor will eventually be
		 * seen.
		 */
		vdo_funnel_queue_put(queue, &queue->stub);

		/* Check again for a successor. */
		next = READ_ONCE(oldest->next);
		if (next == NULL) {
			/*
			 * We lost a race with a producer who swapped queue->newest before we did,
			 * but who hasn't yet updated previous->next. Try again later.
			 */
			return NULL;
		}
	}

	return oldest;
}

/*
 * Poll a queue, removing the oldest entry if the queue is not empty. This function must only be
 * called from a single consumer thread.
 */
struct funnel_queue_entry *vdo_funnel_queue_poll(struct funnel_queue *queue)
{
	struct funnel_queue_entry *oldest = get_oldest(queue);

	if (oldest == NULL)
		return oldest;

	/*
	 * Dequeue the oldest entry and return it. Only one consumer thread may call this function,
	 * so no locking, atomic operations, or fences are needed; queue->oldest is owned by the
	 * consumer and oldest->next is never used by a producer thread after it is swung from NULL
	 * to non-NULL.
	 */
	queue->oldest = READ_ONCE(oldest->next);
	/*
	 * Make sure the caller sees the proper stored data for this entry. Since we've already
	 * fetched the entry pointer we stored in "queue->oldest", this also ensures that on entry
	 * to the next call we'll properly see the dependent data.
	 */
	smp_rmb();
	/*
	 * If "oldest" is a very light-weight work item, we'll be looking for the next one very
	 * soon, so prefetch it now.
	 */
	uds_prefetch_address(queue->oldest, true);
	WRITE_ONCE(oldest->next, NULL);
	return oldest;
}

/*
 * Check whether the funnel queue is empty or not. If the queue is in a transition state with one
 * or more entries being added such that the list view is incomplete, this function will report the
 * queue as empty.
 */
bool vdo_is_funnel_queue_empty(struct funnel_queue *queue)
{
	return get_oldest(queue) == NULL;
}

/*
 * Check whether the funnel queue is idle or not. If the queue has entries available to be
 * retrieved, it is not idle. If the queue is in a transition state with one or more entries being
 * added such that the list view is incomplete, it may not be possible to retrieve an entry with
 * the vdo_funnel_queue_poll() function, but the queue will not be considered idle.
 */
bool vdo_is_funnel_queue_idle(struct funnel_queue *queue)
{
	/*
	 * Oldest is not the stub, so there's another entry, though if next is NULL we can't
	 * retrieve it yet.
	 */
	if (queue->oldest != &queue->stub)
		return false;

	/*
	 * Oldest is the stub, but newest has been updated by _put(); either there's another,
	 * retrievable entry in the list, or the list is officially empty but in the intermediate
	 * state of having an entry added.
	 *
	 * Whether anything is retrievable depends on whether stub.next has been updated and become
	 * visible to us, but for idleness we don't care. And due to memory ordering in _put(), the
	 * update to newest would be visible to us at the same time or sooner.
	 */
	if (READ_ONCE(queue->newest) != &queue->stub)
		return false;

	return true;
}