/*
 * Multipath support for RPC
 *
 * Copyright (c) 2015, 2016, Primary Data, Inc. All rights reserved.
 *
 * Trond Myklebust <trond.myklebust@primarydata.com>
 *
 */
#include <linux/types.h>
#include <linux/kref.h>
#include <linux/list.h>
#include <linux/rcupdate.h>
#include <linux/rculist.h>
#include <linux/slab.h>
#include <asm/cmpxchg.h>
#include <linux/spinlock.h>
#include <linux/sunrpc/xprt.h>
#include <linux/sunrpc/xprtmultipath.h>

typedef struct rpc_xprt *(*xprt_switch_find_xprt_t)(struct list_head *head,
		const struct rpc_xprt *cur);

static const struct rpc_xprt_iter_ops rpc_xprt_iter_singular;
static const struct rpc_xprt_iter_ops rpc_xprt_iter_roundrobin;
static const struct rpc_xprt_iter_ops rpc_xprt_iter_listall;

static void xprt_switch_add_xprt_locked(struct rpc_xprt_switch *xps,
		struct rpc_xprt *xprt)
{
	if (unlikely(xprt_get(xprt) == NULL))
		return;
	list_add_tail_rcu(&xprt->xprt_switch, &xps->xps_xprt_list);
	smp_wmb();
	if (xps->xps_nxprts == 0)
		xps->xps_net = xprt->xprt_net;
	xps->xps_nxprts++;
}

/**
 * rpc_xprt_switch_add_xprt - Add a new rpc_xprt to an rpc_xprt_switch
 * @xps: pointer to struct rpc_xprt_switch
 * @xprt: pointer to struct rpc_xprt
 *
 * Adds xprt to the end of the list of struct rpc_xprt in xps.
 */
void rpc_xprt_switch_add_xprt(struct rpc_xprt_switch *xps,
		struct rpc_xprt *xprt)
{
	if (xprt == NULL)
		return;
	spin_lock(&xps->xps_lock);
	if (xps->xps_net == xprt->xprt_net || xps->xps_net == NULL)
		xprt_switch_add_xprt_locked(xps, xprt);
	spin_unlock(&xps->xps_lock);
}

static void xprt_switch_remove_xprt_locked(struct rpc_xprt_switch *xps,
		struct rpc_xprt *xprt)
{
	if (unlikely(xprt == NULL))
		return;
	xps->xps_nxprts--;
	if (xps->xps_nxprts == 0)
		xps->xps_net = NULL;
	smp_wmb();
	list_del_rcu(&xprt->xprt_switch);
}

/**
 * rpc_xprt_switch_remove_xprt - Removes an rpc_xprt from a rpc_xprt_switch
 * @xps: pointer to struct rpc_xprt_switch
 * @xprt: pointer to struct rpc_xprt
 *
 * Removes xprt from the list of struct rpc_xprt in xps.
 */
void rpc_xprt_switch_remove_xprt(struct rpc_xprt_switch *xps,
		struct rpc_xprt *xprt)
{
	spin_lock(&xps->xps_lock);
	xprt_switch_remove_xprt_locked(xps, xprt);
	spin_unlock(&xps->xps_lock);
	xprt_put(xprt);
}

/**
 * xprt_switch_alloc - Allocate a new struct rpc_xprt_switch
 * @xprt: pointer to struct rpc_xprt
 * @gfp_flags: allocation flags
 *
 * On success, returns an initialised struct rpc_xprt_switch, containing
 * the entry xprt. Returns NULL on failure.
 */
struct rpc_xprt_switch *xprt_switch_alloc(struct rpc_xprt *xprt,
		gfp_t gfp_flags)
{
	struct rpc_xprt_switch *xps;

	xps = kmalloc(sizeof(*xps), gfp_flags);
	if (xps != NULL) {
		spin_lock_init(&xps->xps_lock);
		kref_init(&xps->xps_kref);
		xps->xps_nxprts = 0;
		INIT_LIST_HEAD(&xps->xps_xprt_list);
		xps->xps_iter_ops = &rpc_xprt_iter_singular;
		xprt_switch_add_xprt_locked(xps, xprt);
	}

	return xps;
}

static void xprt_switch_free_entries(struct rpc_xprt_switch *xps)
{
	spin_lock(&xps->xps_lock);
	while (!list_empty(&xps->xps_xprt_list)) {
		struct rpc_xprt *xprt;

		xprt = list_first_entry(&xps->xps_xprt_list,
				struct rpc_xprt, xprt_switch);
		xprt_switch_remove_xprt_locked(xps, xprt);
		spin_unlock(&xps->xps_lock);
		xprt_put(xprt);
		spin_lock(&xps->xps_lock);
	}
	spin_unlock(&xps->xps_lock);
}

static void xprt_switch_free(struct kref *kref)
{
	struct rpc_xprt_switch *xps = container_of(kref,
			struct rpc_xprt_switch, xps_kref);

	xprt_switch_free_entries(xps);
	kfree_rcu(xps, xps_rcu);
}

/**
 * xprt_switch_get - Return a reference to a rpc_xprt_switch
 * @xps: pointer to struct rpc_xprt_switch
 *
 * Returns a reference to xps unless the refcount is already zero.
 */
struct rpc_xprt_switch *xprt_switch_get(struct rpc_xprt_switch *xps)
{
	if (xps != NULL && kref_get_unless_zero(&xps->xps_kref))
		return xps;
	return NULL;
}

/**
 * xprt_switch_put - Release a reference to a rpc_xprt_switch
 * @xps: pointer to struct rpc_xprt_switch
 *
 * Release the reference to xps, and free it once the refcount is zero.
 */
void xprt_switch_put(struct rpc_xprt_switch *xps)
{
	if (xps != NULL)
		kref_put(&xps->xps_kref, xprt_switch_free);
}

/**
 * rpc_xprt_switch_set_roundrobin - Set a round-robin policy on rpc_xprt_switch
 * @xps: pointer to struct rpc_xprt_switch
 *
 * Sets a round-robin default policy for iterators acting on xps.
 */
void rpc_xprt_switch_set_roundrobin(struct rpc_xprt_switch *xps)
{
	if (READ_ONCE(xps->xps_iter_ops) != &rpc_xprt_iter_roundrobin)
		WRITE_ONCE(xps->xps_iter_ops, &rpc_xprt_iter_roundrobin);
}

static
const struct rpc_xprt_iter_ops *xprt_iter_ops(const struct rpc_xprt_iter *xpi)
{
	if (xpi->xpi_ops != NULL)
		return xpi->xpi_ops;
	return rcu_dereference(xpi->xpi_xpswitch)->xps_iter_ops;
}

static
void xprt_iter_no_rewind(struct rpc_xprt_iter *xpi)
{
}

static
void xprt_iter_default_rewind(struct rpc_xprt_iter *xpi)
{
	WRITE_ONCE(xpi->xpi_cursor, NULL);
}

static
struct rpc_xprt *xprt_switch_find_first_entry(struct list_head *head)
{
	return list_first_or_null_rcu(head, struct rpc_xprt, xprt_switch);
}

static
struct rpc_xprt *xprt_iter_first_entry(struct rpc_xprt_iter *xpi)
{
	struct rpc_xprt_switch *xps = rcu_dereference(xpi->xpi_xpswitch);

	if (xps == NULL)
		return NULL;
	return xprt_switch_find_first_entry(&xps->xps_xprt_list);
}

static
struct rpc_xprt *xprt_switch_find_current_entry(struct list_head *head,
		const struct rpc_xprt *cur)
{
	struct rpc_xprt *pos;

	list_for_each_entry_rcu(pos, head, xprt_switch) {
		if (cur == pos)
			return pos;
	}
	return NULL;
}

static
struct rpc_xprt *xprt_iter_current_entry(struct rpc_xprt_iter *xpi)
{
	struct rpc_xprt_switch *xps = rcu_dereference(xpi->xpi_xpswitch);
	struct list_head *head;

	if (xps == NULL)
		return NULL;
	head = &xps->xps_xprt_list;
	if (xpi->xpi_cursor == NULL || xps->xps_nxprts < 2)
		return xprt_switch_find_first_entry(head);
	return xprt_switch_find_current_entry(head, xpi->xpi_cursor);
}

static
struct rpc_xprt *xprt_switch_find_next_entry(struct list_head *head,
		const struct rpc_xprt *cur)
{
	struct rpc_xprt *pos, *prev = NULL;

	list_for_each_entry_rcu(pos, head, xprt_switch) {
		if (cur == prev)
			return pos;
		prev = pos;
	}
	return NULL;
}

static
struct rpc_xprt *xprt_switch_set_next_cursor(struct list_head *head,
		struct rpc_xprt **cursor,
		xprt_switch_find_xprt_t find_next)
{
	struct rpc_xprt *cur, *pos, *old;

	cur = READ_ONCE(*cursor);
	for (;;) {
		old = cur;
		pos = find_next(head, old);
		if (pos == NULL)
			break;
		cur = cmpxchg_relaxed(cursor, old, pos);
		if (cur == old)
			break;
	}
	return pos;
}

static
struct rpc_xprt *xprt_iter_next_entry_multiple(struct rpc_xprt_iter *xpi,
		xprt_switch_find_xprt_t find_next)
{
	struct rpc_xprt_switch *xps = rcu_dereference(xpi->xpi_xpswitch);
	struct list_head *head;

	if (xps == NULL)
		return NULL;
	head = &xps->xps_xprt_list;
	if (xps->xps_nxprts < 2)
		return xprt_switch_find_first_entry(head);
	return xprt_switch_set_next_cursor(head, &xpi->xpi_cursor, find_next);
}

static
struct rpc_xprt *xprt_switch_find_next_entry_roundrobin(struct list_head *head,
		const struct rpc_xprt *cur)
{
	struct rpc_xprt *ret;

	ret = xprt_switch_find_next_entry(head, cur);
	if (ret != NULL)
		return ret;
	return xprt_switch_find_first_entry(head);
}

static
struct rpc_xprt *xprt_iter_next_entry_roundrobin(struct rpc_xprt_iter *xpi)
{
	return xprt_iter_next_entry_multiple(xpi,
			xprt_switch_find_next_entry_roundrobin);
}

static
struct rpc_xprt *xprt_iter_next_entry_all(struct rpc_xprt_iter *xpi)
{
	return xprt_iter_next_entry_multiple(xpi, xprt_switch_find_next_entry);
}

/*
 * xprt_iter_rewind - Resets the xprt iterator
 * @xpi: pointer to rpc_xprt_iter
 *
 * Resets xpi to ensure that it points to the first entry in the list
 * of transports.
 */
static
void xprt_iter_rewind(struct rpc_xprt_iter *xpi)
{
	rcu_read_lock();
	xprt_iter_ops(xpi)->xpi_rewind(xpi);
	rcu_read_unlock();
}

static void __xprt_iter_init(struct rpc_xprt_iter *xpi,
		struct rpc_xprt_switch *xps,
		const struct rpc_xprt_iter_ops *ops)
{
	rcu_assign_pointer(xpi->xpi_xpswitch, xprt_switch_get(xps));
	xpi->xpi_cursor = NULL;
	xpi->xpi_ops = ops;
}

/**
 * xprt_iter_init - Initialise an xprt iterator
 * @xpi: pointer to rpc_xprt_iter
 * @xps: pointer to rpc_xprt_switch
 *
 * Initialises the iterator to use the default iterator ops
 * as set in xps. This function is mainly intended for internal
 * use in the rpc_client.
 */
void xprt_iter_init(struct rpc_xprt_iter *xpi,
		struct rpc_xprt_switch *xps)
{
	__xprt_iter_init(xpi, xps, NULL);
}

/**
 * xprt_iter_init_listall - Initialise an xprt iterator
 * @xpi: pointer to rpc_xprt_iter
 * @xps: pointer to rpc_xprt_switch
 *
 * Initialises the iterator to iterate once through the entire list
 * of entries in xps.
 */
void xprt_iter_init_listall(struct rpc_xprt_iter *xpi,
		struct rpc_xprt_switch *xps)
{
	__xprt_iter_init(xpi, xps, &rpc_xprt_iter_listall);
}

/**
 * xprt_iter_xchg_switch - Atomically swap out the rpc_xprt_switch
 * @xpi: pointer to rpc_xprt_iter
 * @xps: pointer to a new rpc_xprt_switch or NULL
 *
 * Swaps out the existing xpi->xpi_xpswitch with a new value.
 */
struct rpc_xprt_switch *xprt_iter_xchg_switch(struct rpc_xprt_iter *xpi,
		struct rpc_xprt_switch *newswitch)
{
	struct rpc_xprt_switch __rcu *oldswitch;

	/* Atomically swap out the old xpswitch */
	oldswitch = xchg(&xpi->xpi_xpswitch, RCU_INITIALIZER(newswitch));
	if (newswitch != NULL)
		xprt_iter_rewind(xpi);
	return rcu_dereference_protected(oldswitch, true);
}

/**
 * xprt_iter_destroy - Destroys the xprt iterator
 * @xpi pointer to rpc_xprt_iter
 */
void xprt_iter_destroy(struct rpc_xprt_iter *xpi)
{
	xprt_switch_put(xprt_iter_xchg_switch(xpi, NULL));
}

/**
 * xprt_iter_xprt - Returns the rpc_xprt pointed to by the cursor
 * @xpi: pointer to rpc_xprt_iter
 *
 * Returns a pointer to the struct rpc_xprt that is currently
 * pointed to by the cursor.
 * Caller must be holding rcu_read_lock().
 */
struct rpc_xprt *xprt_iter_xprt(struct rpc_xprt_iter *xpi)
{
	WARN_ON_ONCE(!rcu_read_lock_held());
	return xprt_iter_ops(xpi)->xpi_xprt(xpi);
}

static
struct rpc_xprt *xprt_iter_get_helper(struct rpc_xprt_iter *xpi,
		struct rpc_xprt *(*fn)(struct rpc_xprt_iter *))
{
	struct rpc_xprt *ret;

	do {
		ret = fn(xpi);
		if (ret == NULL)
			break;
		ret = xprt_get(ret);
	} while (ret == NULL);
	return ret;
}

/**
 * xprt_iter_get_xprt - Returns the rpc_xprt pointed to by the cursor
 * @xpi: pointer to rpc_xprt_iter
 *
 * Returns a reference to the struct rpc_xprt that is currently
 * pointed to by the cursor.
 */
struct rpc_xprt *xprt_iter_get_xprt(struct rpc_xprt_iter *xpi)
{
	struct rpc_xprt *xprt;

	rcu_read_lock();
	xprt = xprt_iter_get_helper(xpi, xprt_iter_ops(xpi)->xpi_xprt);
	rcu_read_unlock();
	return xprt;
}

/**
 * xprt_iter_get_next - Returns the next rpc_xprt following the cursor
 * @xpi: pointer to rpc_xprt_iter
 *
 * Returns a reference to the struct rpc_xprt that immediately follows the
 * entry pointed to by the cursor.
 */
struct rpc_xprt *xprt_iter_get_next(struct rpc_xprt_iter *xpi)
{
	struct rpc_xprt *xprt;

	rcu_read_lock();
	xprt = xprt_iter_get_helper(xpi, xprt_iter_ops(xpi)->xpi_next);
	rcu_read_unlock();
	return xprt;
}

/* Policy for always returning the first entry in the rpc_xprt_switch */
static
const struct rpc_xprt_iter_ops rpc_xprt_iter_singular = {
	.xpi_rewind = xprt_iter_no_rewind,
	.xpi_xprt = xprt_iter_first_entry,
	.xpi_next = xprt_iter_first_entry,
};

/* Policy for round-robin iteration of entries in the rpc_xprt_switch */
static
const struct rpc_xprt_iter_ops rpc_xprt_iter_roundrobin = {
	.xpi_rewind = xprt_iter_default_rewind,
	.xpi_xprt = xprt_iter_current_entry,
	.xpi_next = xprt_iter_next_entry_roundrobin,
};

/* Policy for once-through iteration of entries in the rpc_xprt_switch */
static
const struct rpc_xprt_iter_ops rpc_xprt_iter_listall = {
	.xpi_rewind = xprt_iter_default_rewind,
	.xpi_xprt = xprt_iter_current_entry,
	.xpi_next = xprt_iter_next_entry_all,
};