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/kernel.h>
#include <linux/rcupdate.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
 *
 * Errors are also represented as internal entries, but use the negative
 * space (-4094 to -2).  They're never stored in the slots array; only
 * returned by the normal API.
 */

#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;
}

/**
 * xa_is_err() - Report whether an XArray operation returned an error
 * @entry: Result from calling an XArray function
 *
 * If an XArray operation cannot complete an operation, it will return
 * a special value indicating an error.  This function tells you
 * whether an error occurred; xa_err() tells you which error occurred.
 *
 * Context: Any context.
 * Return: %true if the entry indicates an error.
 */
static inline bool xa_is_err(const void *entry)
{
	return unlikely(xa_is_internal(entry));
}

/**
 * xa_err() - Turn an XArray result into an errno.
 * @entry: Result from calling an XArray function.
 *
 * If an XArray operation cannot complete an operation, it will return
 * a special pointer value which encodes an errno.  This function extracts
 * the errno from the pointer value, or returns 0 if the pointer does not
 * represent an errno.
 *
 * Context: Any context.
 * Return: A negative errno or 0.
 */
static inline int xa_err(void *entry)
{
	/* xa_to_internal() would not do sign extension. */
	if (xa_is_err(entry))
		return (long)entry >> 2;
	return 0;
}

/**
 * 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);
void *xa_load(struct xarray *, unsigned long index);

/**
 * 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);
}

/**
 * xa_empty() - Determine if an array has any present entries.
 * @xa: XArray.
 *
 * Context: Any context.
 * Return: %true if the array contains only NULL pointers.
 */
static inline bool xa_empty(const struct xarray *xa)
{
	return xa->xa_head == NULL;
}

#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)
#define XA_MAX_MARKS		3
#define XA_MARK_LONGS		DIV_ROUND_UP(XA_CHUNK_SIZE, BITS_PER_LONG)

/*
 * @count is the count of every non-NULL element in the ->slots array
 * whether that is a value entry, a retry entry, a user pointer,
 * a sibling entry or a pointer to the next level of the tree.
 * @nr_values is the count of every element in ->slots which is
 * either a value entry or a sibling of a value entry.
 */
struct xa_node {
	unsigned char	shift;		/* Bits remaining in each slot */
	unsigned char	offset;		/* Slot offset in parent */
	unsigned char	count;		/* Total entry count */
	unsigned char	nr_values;	/* Value entry count */
	struct xa_node __rcu *parent;	/* NULL at top of tree */
	struct xarray	*array;		/* The array we belong to */
	union {
		struct list_head private_list;	/* For tree user */
		struct rcu_head	rcu_head;	/* Used when freeing node */
	};
	void __rcu	*slots[XA_CHUNK_SIZE];
	union {
		unsigned long	tags[XA_MAX_MARKS][XA_MARK_LONGS];
		unsigned long	marks[XA_MAX_MARKS][XA_MARK_LONGS];
	};
};

void xa_dump(const struct xarray *);
void xa_dump_node(const struct xa_node *);

#ifdef XA_DEBUG
#define XA_BUG_ON(xa, x) do {					\
		if (x) {					\
			xa_dump(xa);				\
			BUG();					\
		}						\
	} while (0)
#define XA_NODE_BUG_ON(node, x) do {				\
		if (x) {					\
			if (node) xa_dump_node(node);		\
			BUG();					\
		}						\
	} while (0)
#else
#define XA_BUG_ON(xa, x)	do { } while (0)
#define XA_NODE_BUG_ON(node, x)	do { } while (0)
#endif

/* Private */
static inline void *xa_head(const struct xarray *xa)
{
	return rcu_dereference_check(xa->xa_head,
						lockdep_is_held(&xa->xa_lock));
}

/* Private */
static inline void *xa_head_locked(const struct xarray *xa)
{
	return rcu_dereference_protected(xa->xa_head,
						lockdep_is_held(&xa->xa_lock));
}

/* Private */
static inline void *xa_entry(const struct xarray *xa,
				const struct xa_node *node, unsigned int offset)
{
	XA_NODE_BUG_ON(node, offset >= XA_CHUNK_SIZE);
	return rcu_dereference_check(node->slots[offset],
						lockdep_is_held(&xa->xa_lock));
}

/* Private */
static inline void *xa_entry_locked(const struct xarray *xa,
				const struct xa_node *node, unsigned int offset)
{
	XA_NODE_BUG_ON(node, offset >= XA_CHUNK_SIZE);
	return rcu_dereference_protected(node->slots[offset],
						lockdep_is_held(&xa->xa_lock));
}

/* Private */
static inline struct xa_node *xa_to_node(const void *entry)
{
	return (struct xa_node *)((unsigned long)entry - 2);
}

/* 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)

/**
 * xa_is_retry() - Is the entry a retry entry?
 * @entry: Entry retrieved from the XArray
 *
 * Return: %true if the entry is a retry entry.
 */
static inline bool xa_is_retry(const void *entry)
{
	return unlikely(entry == XA_RETRY_ENTRY);
}

/**
 * typedef xa_update_node_t - A callback function from the XArray.
 * @node: The node which is being processed
 *
 * This function is called every time the XArray updates the count of
 * present and value entries in a node.  It allows advanced users to
 * maintain the private_list in the node.
 *
 * Context: The xa_lock is held and interrupts may be disabled.
 *	    Implementations should not drop the xa_lock, nor re-enable
 *	    interrupts.
 */
typedef void (*xa_update_node_t)(struct xa_node *node);

/*
 * The xa_state is opaque to its users.  It contains various different pieces
 * of state involved in the current operation on the XArray.  It should be
 * declared on the stack and passed between the various internal routines.
 * The various elements in it should not be accessed directly, but only
 * through the provided accessor functions.  The below documentation is for
 * the benefit of those working on the code, not for users of the XArray.
 *
 * @xa_node usually points to the xa_node containing the slot we're operating
 * on (and @xa_offset is the offset in the slots array).  If there is a
 * single entry in the array at index 0, there are no allocated xa_nodes to
 * point to, and so we store %NULL in @xa_node.  @xa_node is set to
 * the value %XAS_RESTART if the xa_state is not walked to the correct
 * position in the tree of nodes for this operation.  If an error occurs
 * during an operation, it is set to an %XAS_ERROR value.  If we run off the
 * end of the allocated nodes, it is set to %XAS_BOUNDS.
 */
struct xa_state {
	struct xarray *xa;
	unsigned long xa_index;
	unsigned char xa_shift;
	unsigned char xa_sibs;
	unsigned char xa_offset;
	unsigned char xa_pad;		/* Helps gcc generate better code */
	struct xa_node *xa_node;
	struct xa_node *xa_alloc;
	xa_update_node_t xa_update;
};

/*
 * We encode errnos in the xas->xa_node.  If an error has happened, we need to
 * drop the lock to fix it, and once we've done so the xa_state is invalid.
 */
#define XA_ERROR(errno) ((struct xa_node *)(((unsigned long)errno << 2) | 2UL))
#define XAS_BOUNDS	((struct xa_node *)1UL)
#define XAS_RESTART	((struct xa_node *)3UL)

#define __XA_STATE(array, index, shift, sibs)  {	\
	.xa = array,					\
	.xa_index = index,				\
	.xa_shift = shift,				\
	.xa_sibs = sibs,				\
	.xa_offset = 0,					\
	.xa_pad = 0,					\
	.xa_node = XAS_RESTART,				\
	.xa_alloc = NULL,				\
	.xa_update = NULL				\
}

/**
 * XA_STATE() - Declare an XArray operation state.
 * @name: Name of this operation state (usually xas).
 * @array: Array to operate on.
 * @index: Initial index of interest.
 *
 * Declare and initialise an xa_state on the stack.
 */
#define XA_STATE(name, array, index)				\
	struct xa_state name = __XA_STATE(array, index, 0, 0)

/**
 * XA_STATE_ORDER() - Declare an XArray operation state.
 * @name: Name of this operation state (usually xas).
 * @array: Array to operate on.
 * @index: Initial index of interest.
 * @order: Order of entry.
 *
 * Declare and initialise an xa_state on the stack.  This variant of
 * XA_STATE() allows you to specify the 'order' of the element you
 * want to operate on.`
 */
#define XA_STATE_ORDER(name, array, index, order)		\
	struct xa_state name = __XA_STATE(array,		\
			(index >> order) << order,		\
			order - (order % XA_CHUNK_SHIFT),	\
			(1U << (order % XA_CHUNK_SHIFT)) - 1)

#define xas_marked(xas, mark)	xa_marked((xas)->xa, (mark))
#define xas_trylock(xas)	xa_trylock((xas)->xa)
#define xas_lock(xas)		xa_lock((xas)->xa)
#define xas_unlock(xas)		xa_unlock((xas)->xa)
#define xas_lock_bh(xas)	xa_lock_bh((xas)->xa)
#define xas_unlock_bh(xas)	xa_unlock_bh((xas)->xa)
#define xas_lock_irq(xas)	xa_lock_irq((xas)->xa)
#define xas_unlock_irq(xas)	xa_unlock_irq((xas)->xa)
#define xas_lock_irqsave(xas, flags) \
				xa_lock_irqsave((xas)->xa, flags)
#define xas_unlock_irqrestore(xas, flags) \
				xa_unlock_irqrestore((xas)->xa, flags)

/**
 * xas_error() - Return an errno stored in the xa_state.
 * @xas: XArray operation state.
 *
 * Return: 0 if no error has been noted.  A negative errno if one has.
 */
static inline int xas_error(const struct xa_state *xas)
{
	return xa_err(xas->xa_node);
}

/**
 * xas_set_err() - Note an error in the xa_state.
 * @xas: XArray operation state.
 * @err: Negative error number.
 *
 * Only call this function with a negative @err; zero or positive errors
 * will probably not behave the way you think they should.  If you want
 * to clear the error from an xa_state, use xas_reset().
 */
static inline void xas_set_err(struct xa_state *xas, long err)
{
	xas->xa_node = XA_ERROR(err);
}

/**
 * xas_invalid() - Is the xas in a retry or error state?
 * @xas: XArray operation state.
 *
 * Return: %true if the xas cannot be used for operations.
 */
static inline bool xas_invalid(const struct xa_state *xas)
{
	return (unsigned long)xas->xa_node & 3;
}

/**
 * xas_valid() - Is the xas a valid cursor into the array?
 * @xas: XArray operation state.
 *
 * Return: %true if the xas can be used for operations.
 */
static inline bool xas_valid(const struct xa_state *xas)
{
	return !xas_invalid(xas);
}

/**
 * xas_reset() - Reset an XArray operation state.
 * @xas: XArray operation state.
 *
 * Resets the error or walk state of the @xas so future walks of the
 * array will start from the root.  Use this if you have dropped the
 * xarray lock and want to reuse the xa_state.
 *
 * Context: Any context.
 */
static inline void xas_reset(struct xa_state *xas)
{
	xas->xa_node = XAS_RESTART;
}

/**
 * xas_retry() - Retry the operation if appropriate.
 * @xas: XArray operation state.
 * @entry: Entry from xarray.
 *
 * The advanced functions may sometimes return an internal entry, such as
 * a retry entry or a zero entry.  This function sets up the @xas to restart
 * the walk from the head of the array if needed.
 *
 * Context: Any context.
 * Return: true if the operation needs to be retried.
 */
static inline bool xas_retry(struct xa_state *xas, const void *entry)
{
	if (!xa_is_retry(entry))
		return false;
	xas_reset(xas);
	return true;
}

void *xas_load(struct xa_state *);

/**
 * xas_reload() - Refetch an entry from the xarray.
 * @xas: XArray operation state.
 *
 * Use this function to check that a previously loaded entry still has
 * the same value.  This is useful for the lockless pagecache lookup where
 * we walk the array with only the RCU lock to protect us, lock the page,
 * then check that the page hasn't moved since we looked it up.
 *
 * The caller guarantees that @xas is still valid.  If it may be in an
 * error or restart state, call xas_load() instead.
 *
 * Return: The entry at this location in the xarray.
 */
static inline void *xas_reload(struct xa_state *xas)
{
	struct xa_node *node = xas->xa_node;

	if (node)
		return xa_entry(xas->xa, node, xas->xa_offset);
	return xa_head(xas->xa);
}

#endif /* _LINUX_XARRAY_H */