include/linux/xarray.h

/* SPDX-License-Identifier: GPL-2.0+ */
#ifndef _LINUX_XARRAY_H
#define _LINUX_XARRAY_H
/*
 * eXtensible Arrays
 * Copyright (c) 2017 Microsoft Corporation
 * Author: Matthew Wilcox <willy@infradead.org>
 *
 * See Documentation/core-api/xarray.rst for how to use the XArray.
 */

#include <linux/bug.h>
#include <linux/compiler.h>
#include <linux/kconfig.h>
#include <linux/spinlock.h>
#include <linux/types.h>

/*
 * The bottom two bits of the entry determine how the XArray interprets
 * the contents:
 *
 * 00: Pointer entry
 * 10: Internal entry
 * x1: Value entry or tagged pointer
 *
 * Attempting to store internal entries in the XArray is a bug.
 *
 * Most internal entries are pointers to the next node in the tree.
 * The following internal entries have a special meaning:
 *
 * 0-62: Sibling entries
 * 256: Retry entry
 */

#define BITS_PER_XA_VALUE	(BITS_PER_LONG - 1)

/**
 * xa_mk_value() - Create an XArray entry from an integer.
 * @v: Value to store in XArray.
 *
 * Context: Any context.
 * Return: An entry suitable for storing in the XArray.
 */
static inline void *xa_mk_value(unsigned long v)
{
	WARN_ON((long)v < 0);
	return (void *)((v << 1) | 1);
}

/**
 * xa_to_value() - Get value stored in an XArray entry.
 * @entry: XArray entry.
 *
 * Context: Any context.
 * Return: The value stored in the XArray entry.
 */
static inline unsigned long xa_to_value(const void *entry)
{
	return (unsigned long)entry >> 1;
}

/**
 * xa_is_value() - Determine if an entry is a value.
 * @entry: XArray entry.
 *
 * Context: Any context.
 * Return: True if the entry is a value, false if it is a pointer.
 */
static inline bool xa_is_value(const void *entry)
{
	return (unsigned long)entry & 1;
}

/**
 * xa_tag_pointer() - Create an XArray entry for a tagged pointer.
 * @p: Plain pointer.
 * @tag: Tag value (0, 1 or 3).
 *
 * If the user of the XArray prefers, they can tag their pointers instead
 * of storing value entries.  Three tags are available (0, 1 and 3).
 * These are distinct from the xa_mark_t as they are not replicated up
 * through the array and cannot be searched for.
 *
 * Context: Any context.
 * Return: An XArray entry.
 */
static inline void *xa_tag_pointer(void *p, unsigned long tag)
{
	return (void *)((unsigned long)p | tag);
}

/**
 * xa_untag_pointer() - Turn an XArray entry into a plain pointer.
 * @entry: XArray entry.
 *
 * If you have stored a tagged pointer in the XArray, call this function
 * to get the untagged version of the pointer.
 *
 * Context: Any context.
 * Return: A pointer.
 */
static inline void *xa_untag_pointer(void *entry)
{
	return (void *)((unsigned long)entry & ~3UL);
}

/**
 * xa_pointer_tag() - Get the tag stored in an XArray entry.
 * @entry: XArray entry.
 *
 * If you have stored a tagged pointer in the XArray, call this function
 * to get the tag of that pointer.
 *
 * Context: Any context.
 * Return: A tag.
 */
static inline unsigned int xa_pointer_tag(void *entry)
{
	return (unsigned long)entry & 3UL;
}

/*
 * xa_mk_internal() - Create an internal entry.
 * @v: Value to turn into an internal entry.
 *
 * Context: Any context.
 * Return: An XArray internal entry corresponding to this value.
 */
static inline void *xa_mk_internal(unsigned long v)
{
	return (void *)((v << 2) | 2);
}

/*
 * xa_to_internal() - Extract the value from an internal entry.
 * @entry: XArray entry.
 *
 * Context: Any context.
 * Return: The value which was stored in the internal entry.
 */
static inline unsigned long xa_to_internal(const void *entry)
{
	return (unsigned long)entry >> 2;
}

/*
 * xa_is_internal() - Is the entry an internal entry?
 * @entry: XArray entry.
 *
 * Context: Any context.
 * Return: %true if the entry is an internal entry.
 */
static inline bool xa_is_internal(const void *entry)
{
	return ((unsigned long)entry & 3) == 2;
}

/**
 * struct xarray - The anchor of the XArray.
 * @xa_lock: Lock that protects the contents of the XArray.
 *
 * To use the xarray, define it statically or embed it in your data structure.
 * It is a very small data structure, so it does not usually make sense to
 * allocate it separately and keep a pointer to it in your data structure.
 *
 * You may use the xa_lock to protect your own data structures as well.
 */
/*
 * If all of the entries in the array are NULL, @xa_head is a NULL pointer.
 * If the only non-NULL entry in the array is at index 0, @xa_head is that
 * entry.  If any other entry in the array is non-NULL, @xa_head points
 * to an @xa_node.
 */
struct xarray {
	spinlock_t	xa_lock;
/* private: The rest of the data structure is not to be used directly. */
	gfp_t		xa_flags;
	void __rcu *	xa_head;
};

#define XARRAY_INIT(name, flags) {				\
	.xa_lock = __SPIN_LOCK_UNLOCKED(name.xa_lock),		\
	.xa_flags = flags,					\
	.xa_head = NULL,					\
}

/**
 * DEFINE_XARRAY_FLAGS() - Define an XArray with custom flags.
 * @name: A string that names your XArray.
 * @flags: XA_FLAG values.
 *
 * This is intended for file scope definitions of XArrays.  It declares
 * and initialises an empty XArray with the chosen name and flags.  It is
 * equivalent to calling xa_init_flags() on the array, but it does the
 * initialisation at compiletime instead of runtime.
 */
#define DEFINE_XARRAY_FLAGS(name, flags)				\
	struct xarray name = XARRAY_INIT(name, flags)

/**
 * DEFINE_XARRAY() - Define an XArray.
 * @name: A string that names your XArray.
 *
 * This is intended for file scope definitions of XArrays.  It declares
 * and initialises an empty XArray with the chosen name.  It is equivalent
 * to calling xa_init() on the array, but it does the initialisation at
 * compiletime instead of runtime.
 */
#define DEFINE_XARRAY(name) DEFINE_XARRAY_FLAGS(name, 0)

void xa_init_flags(struct xarray *, gfp_t flags);

/**
 * xa_init() - Initialise an empty XArray.
 * @xa: XArray.
 *
 * An empty XArray is full of NULL entries.
 *
 * Context: Any context.
 */
static inline void xa_init(struct xarray *xa)
{
	xa_init_flags(xa, 0);
}

#define xa_trylock(xa)		spin_trylock(&(xa)->xa_lock)
#define xa_lock(xa)		spin_lock(&(xa)->xa_lock)
#define xa_unlock(xa)		spin_unlock(&(xa)->xa_lock)
#define xa_lock_bh(xa)		spin_lock_bh(&(xa)->xa_lock)
#define xa_unlock_bh(xa)	spin_unlock_bh(&(xa)->xa_lock)
#define xa_lock_irq(xa)		spin_lock_irq(&(xa)->xa_lock)
#define xa_unlock_irq(xa)	spin_unlock_irq(&(xa)->xa_lock)
#define xa_lock_irqsave(xa, flags) \
				spin_lock_irqsave(&(xa)->xa_lock, flags)
#define xa_unlock_irqrestore(xa, flags) \
				spin_unlock_irqrestore(&(xa)->xa_lock, flags)

/* Everything below here is the Advanced API.  Proceed with caution. */

/*
 * The xarray is constructed out of a set of 'chunks' of pointers.  Choosing
 * the best chunk size requires some tradeoffs.  A power of two recommends
 * itself so that we can walk the tree based purely on shifts and masks.
 * Generally, the larger the better; as the number of slots per level of the
 * tree increases, the less tall the tree needs to be.  But that needs to be
 * balanced against the memory consumption of each node.  On a 64-bit system,
 * xa_node is currently 576 bytes, and we get 7 of them per 4kB page.  If we
 * doubled the number of slots per node, we'd get only 3 nodes per 4kB page.
 */
#ifndef XA_CHUNK_SHIFT
#define XA_CHUNK_SHIFT		(CONFIG_BASE_SMALL ? 4 : 6)
#endif
#define XA_CHUNK_SIZE		(1UL << XA_CHUNK_SHIFT)
#define XA_CHUNK_MASK		(XA_CHUNK_SIZE - 1)

/* Private */
static inline bool xa_is_node(const void *entry)
{
	return xa_is_internal(entry) && (unsigned long)entry > 4096;
}

/* Private */
static inline void *xa_mk_sibling(unsigned int offset)
{
	return xa_mk_internal(offset);
}

/* Private */
static inline unsigned long xa_to_sibling(const void *entry)
{
	return xa_to_internal(entry);
}

/**
 * xa_is_sibling() - Is the entry a sibling entry?
 * @entry: Entry retrieved from the XArray
 *
 * Return: %true if the entry is a sibling entry.
 */
static inline bool xa_is_sibling(const void *entry)
{
	return IS_ENABLED(CONFIG_XARRAY_MULTI) && xa_is_internal(entry) &&
		(entry < xa_mk_sibling(XA_CHUNK_SIZE - 1));
}

#define XA_RETRY_ENTRY		xa_mk_internal(256)

#endif /* _LINUX_XARRAY_H */