linux/fs/btrfs/ulist.c

/*
 * Copyright (C) 2011 STRATO AG
 * written by Arne Jansen <sensille@gmx.net>
 * Distributed under the GNU GPL license version 2.
 */

#include <linux/slab.h>
#include <linux/module.h>
#include "ulist.h"

/*
 * ulist is a generic data structure to hold a collection of unique u64
 * values. The only operations it supports is adding to the list and
 * enumerating it.
 * It is possible to store an auxiliary value along with the key.
 *
 * The implementation is preliminary and can probably be sped up
 * significantly. A first step would be to store the values in an rbtree
 * as soon as ULIST_SIZE is exceeded.
 *
 * A sample usage for ulists is the enumeration of directed graphs without
 * visiting a node twice. The pseudo-code could look like this:
 *
 * ulist = ulist_alloc();
 * ulist_add(ulist, root);
 * elem = NULL;
 *
 * while ((elem = ulist_next(ulist, elem)) {
 * 	for (all child nodes n in elem)
 *		ulist_add(ulist, n);
 *	do something useful with the node;
 * }
 * ulist_free(ulist);
 *
 * This assumes the graph nodes are adressable by u64. This stems from the
 * usage for tree enumeration in btrfs, where the logical addresses are
 * 64 bit.
 *
 * It is also useful for tree enumeration which could be done elegantly
 * recursively, but is not possible due to kernel stack limitations. The
 * loop would be similar to the above.
 */

/**
 * ulist_init - freshly initialize a ulist
 * @ulist:	the ulist to initialize
 *
 * Note: don't use this function to init an already used ulist, use
 * ulist_reinit instead.
 */
void ulist_init(struct ulist *ulist)
{
	ulist->nnodes = 0;
	ulist->nodes = ulist->int_nodes;
	ulist->nodes_alloced = ULIST_SIZE;
}
EXPORT_SYMBOL(ulist_init);

/**
 * ulist_fini - free up additionally allocated memory for the ulist
 * @ulist:	the ulist from which to free the additional memory
 *
 * This is useful in cases where the base 'struct ulist' has been statically
 * allocated.
 */
void ulist_fini(struct ulist *ulist)
{
	/*
	 * The first ULIST_SIZE elements are stored inline in struct ulist.
	 * Only if more elements are alocated they need to be freed.
	 */
	if (ulist->nodes_alloced > ULIST_SIZE)
		kfree(ulist->nodes);
	ulist->nodes_alloced = 0;	/* in case ulist_fini is called twice */
}
EXPORT_SYMBOL(ulist_fini);

/**
 * ulist_reinit - prepare a ulist for reuse
 * @ulist:	ulist to be reused
 *
 * Free up all additional memory allocated for the list elements and reinit
 * the ulist.
 */
void ulist_reinit(struct ulist *ulist)
{
	ulist_fini(ulist);
	ulist_init(ulist);
}
EXPORT_SYMBOL(ulist_reinit);

/**
 * ulist_alloc - dynamically allocate a ulist
 * @gfp_mask:	allocation flags to for base allocation
 *
 * The allocated ulist will be returned in an initialized state.
 */
struct ulist *ulist_alloc(unsigned long gfp_mask)
{
	struct ulist *ulist = kmalloc(sizeof(*ulist), gfp_mask);

	if (!ulist)
		return NULL;

	ulist_init(ulist);

	return ulist;
}
EXPORT_SYMBOL(ulist_alloc);

/**
 * ulist_free - free dynamically allocated ulist
 * @ulist:	ulist to free
 *
 * It is not necessary to call ulist_fini before.
 */
void ulist_free(struct ulist *ulist)
{
	if (!ulist)
		return;
	ulist_fini(ulist);
	kfree(ulist);
}
EXPORT_SYMBOL(ulist_free);

/**
 * ulist_add - add an element to the ulist
 * @ulist:	ulist to add the element to
 * @val:	value to add to ulist
 * @aux:	auxiliary value to store along with val
 * @gfp_mask:	flags to use for allocation
 *
 * Note: locking must be provided by the caller. In case of rwlocks write
 *       locking is needed
 *
 * Add an element to a ulist. The @val will only be added if it doesn't
 * already exist. If it is added, the auxiliary value @aux is stored along with
 * it. In case @val already exists in the ulist, @aux is ignored, even if
 * it differs from the already stored value.
 *
 * ulist_add returns 0 if @val already exists in ulist and 1 if @val has been
 * inserted.
 * In case of allocation failure -ENOMEM is returned and the ulist stays
 * unaltered.
 */
int ulist_add(struct ulist *ulist, u64 val, unsigned long aux,
	      unsigned long gfp_mask)
{
	int i;

	for (i = 0; i < ulist->nnodes; ++i) {
		if (ulist->nodes[i].val == val)
			return 0;
	}

	if (ulist->nnodes >= ulist->nodes_alloced) {
		u64 new_alloced = ulist->nodes_alloced + 128;
		struct ulist_node *new_nodes;
		void *old = NULL;

		/*
		 * if nodes_alloced == ULIST_SIZE no memory has been allocated
		 * yet, so pass NULL to krealloc
		 */
		if (ulist->nodes_alloced > ULIST_SIZE)
			old = ulist->nodes;

		new_nodes = krealloc(old, sizeof(*new_nodes) * new_alloced,
				     gfp_mask);
		if (!new_nodes)
			return -ENOMEM;

		if (!old)
			memcpy(new_nodes, ulist->int_nodes,
			       sizeof(ulist->int_nodes));

		ulist->nodes = new_nodes;
		ulist->nodes_alloced = new_alloced;
	}
	ulist->nodes[ulist->nnodes].val = val;
	ulist->nodes[ulist->nnodes].aux = aux;
	++ulist->nnodes;

	return 1;
}
EXPORT_SYMBOL(ulist_add);

/**
 * ulist_next - iterate ulist
 * @ulist:	ulist to iterate
 * @prev:	previously returned element or %NULL to start iteration
 *
 * Note: locking must be provided by the caller. In case of rwlocks only read
 *       locking is needed
 *
 * This function is used to iterate an ulist. The iteration is started with
 * @prev = %NULL. It returns the next element from the ulist or %NULL when the
 * end is reached. No guarantee is made with respect to the order in which
 * the elements are returned. They might neither be returned in order of
 * addition nor in ascending order.
 * It is allowed to call ulist_add during an enumeration. Newly added items
 * are guaranteed to show up in the running enumeration.
 */
struct ulist_node *ulist_next(struct ulist *ulist, struct ulist_node *prev)
{
	int next;

	if (ulist->nnodes == 0)
		return NULL;

	if (!prev)
		return &ulist->nodes[0];

	next = (prev - ulist->nodes) + 1;
	if (next < 0 || next >= ulist->nnodes)
		return NULL;

	return &ulist->nodes[next];
}
EXPORT_SYMBOL(ulist_next);