lib/idr.c - fp2-dev/kernel/msm - Gitiles

 /*
  * 2002-10-18  written by Jim Houston jim.houston@ccur.com
  *	Copyright (C) 2002 by Concurrent Computer Corporation
  *	Distributed under the GNU GPL license version 2.
  *
  * Modified by George Anzinger to reuse immediately and to use
  * find bit instructions.  Also removed _irq on spinlocks.
  *
  * Small id to pointer translation service.
  *
  * It uses a radix tree like structure as a sparse array indexed
  * by the id to obtain the pointer.  The bitmap makes allocating
  * a new id quick.
  *
  * You call it to allocate an id (an int) an associate with that id a
  * pointer or what ever, we treat it as a (void *).  You can pass this
  * id to a user for him to pass back at a later time.  You then pass
  * that id to this code and it returns your pointer.

  * You can release ids at any time. When all ids are released, most of
  * the memory is returned (we keep IDR_FREE_MAX) in a local pool so we
  * don't need to go to the memory "store" during an id allocate, just
  * so you don't need to be too concerned about locking and conflicts
  * with the slab allocator.
  */

 #ifndef TEST                        // to test in user space...
 #include <linux/slab.h>
 #include <linux/init.h>
 #include <linux/module.h>
 #endif
 #include <linux/err.h>
 #include <linux/string.h>
 #include <linux/idr.h>

 static struct kmem_cache *idr_layer_cache;

 static struct idr_layer *alloc_layer(struct idr *idp)
 {
 	struct idr_layer *p;
 	unsigned long flags;

 	spin_lock_irqsave(&idp->lock, flags);
 	if ((p = idp->id_free)) {
 		idp->id_free = p->ary[0];
 		idp->id_free_cnt--;
 		p->ary[0] = NULL;
 	}
 	spin_unlock_irqrestore(&idp->lock, flags);
 	return(p);
 }

 /* only called when idp->lock is held */
 static void __free_layer(struct idr *idp, struct idr_layer *p)
 {
 	p->ary[0] = idp->id_free;
 	idp->id_free = p;
 	idp->id_free_cnt++;
 }

 static void free_layer(struct idr *idp, struct idr_layer *p)
 {
 	unsigned long flags;

 	/*
 	 * Depends on the return element being zeroed.
 	 */
 	spin_lock_irqsave(&idp->lock, flags);
 	__free_layer(idp, p);
 	spin_unlock_irqrestore(&idp->lock, flags);
 }

 /**
  * idr_pre_get - reserver resources for idr allocation
  * @idp:	idr handle
  * @gfp_mask:	memory allocation flags
  *
  * This function should be called prior to locking and calling the
  * following function.  It preallocates enough memory to satisfy
  * the worst possible allocation.
  *
  * If the system is REALLY out of memory this function returns 0,
  * otherwise 1.
  */
 int idr_pre_get(struct idr *idp, gfp_t gfp_mask)
 {
 	while (idp->id_free_cnt < IDR_FREE_MAX) {
 		struct idr_layer *new;
 		new = kmem_cache_alloc(idr_layer_cache, gfp_mask);
 		if (new == NULL)
 			return (0);
 		free_layer(idp, new);
 	}
 	return 1;
 }
 EXPORT_SYMBOL(idr_pre_get);

 static int sub_alloc(struct idr *idp, void *ptr, int *starting_id)
 {
 	int n, m, sh;
 	struct idr_layer *p, *new;
 	struct idr_layer *pa[MAX_LEVEL];
 	int l, id, oid;
 	long bm;

 	id = *starting_id;
  restart:
 	p = idp->top;
 	l = idp->layers;
 	pa[l--] = NULL;
 	while (1) {
 		/*
 		 * We run around this while until we reach the leaf node...
 		 */
 		n = (id >> (IDR_BITS*l)) & IDR_MASK;
 		bm = ~p->bitmap;
 		m = find_next_bit(&bm, IDR_SIZE, n);
 		if (m == IDR_SIZE) {
 			/* no space available go back to previous layer. */
 			l++;
 			oid = id;
 			id = (id | ((1 << (IDR_BITS * l)) - 1)) + 1;

 			/* if already at the top layer, we need to grow */
 			if (!(p = pa[l])) {
 				*starting_id = id;
 				return -2;
 			}

 			/* If we need to go up one layer, continue the
 			 * loop; otherwise, restart from the top.
 			 */
 			sh = IDR_BITS * (l + 1);
 			if (oid >> sh == id >> sh)
 				continue;
 			else
 				goto restart;
 		}
 		if (m != n) {
 			sh = IDR_BITS*l;
 			id = ((id >> sh) ^ n ^ m) << sh;
 		}
 		if ((id >= MAX_ID_BIT) || (id < 0))
 			return -3;
 		if (l == 0)
 			break;
 		/*
 		 * Create the layer below if it is missing.
 		 */
 		if (!p->ary[m]) {
 			if (!(new = alloc_layer(idp)))
 				return -1;
 			p->ary[m] = new;
 			p->count++;
 		}
 		pa[l--] = p;
 		p = p->ary[m];
 	}
 	/*
 	 * We have reached the leaf node, plant the
 	 * users pointer and return the raw id.
 	 */
 	p->ary[m] = (struct idr_layer *)ptr;
 	__set_bit(m, &p->bitmap);
 	p->count++;
 	/*
 	 * If this layer is full mark the bit in the layer above
 	 * to show that this part of the radix tree is full.
 	 * This may complete the layer above and require walking
 	 * up the radix tree.
 	 */
 	n = id;
 	while (p->bitmap == IDR_FULL) {
 		if (!(p = pa[++l]))
 			break;
 		n = n >> IDR_BITS;
 		__set_bit((n & IDR_MASK), &p->bitmap);
 	}
 	return(id);
 }

 static int idr_get_new_above_int(struct idr *idp, void *ptr, int starting_id)
 {
 	struct idr_layer *p, *new;
 	int layers, v, id;
 	unsigned long flags;

 	id = starting_id;
 build_up:
 	p = idp->top;
 	layers = idp->layers;
 	if (unlikely(!p)) {
 		if (!(p = alloc_layer(idp)))
 			return -1;
 		layers = 1;
 	}
 	/*
 	 * Add a new layer to the top of the tree if the requested
 	 * id is larger than the currently allocated space.
 	 */
 	while ((layers < (MAX_LEVEL - 1)) && (id >= (1 << (layers*IDR_BITS)))) {
 		layers++;
 		if (!p->count)
 			continue;
 		if (!(new = alloc_layer(idp))) {
 			/*
 			 * The allocation failed.  If we built part of
 			 * the structure tear it down.
 			 */
 			spin_lock_irqsave(&idp->lock, flags);
 			for (new = p; p && p != idp->top; new = p) {
 				p = p->ary[0];
 				new->ary[0] = NULL;
 				new->bitmap = new->count = 0;
 				__free_layer(idp, new);
 			}
 			spin_unlock_irqrestore(&idp->lock, flags);
 			return -1;
 		}
 		new->ary[0] = p;
 		new->count = 1;
 		if (p->bitmap == IDR_FULL)
 			__set_bit(0, &new->bitmap);
 		p = new;
 	}
 	idp->top = p;
 	idp->layers = layers;
 	v = sub_alloc(idp, ptr, &id);
 	if (v == -2)
 		goto build_up;
 	return(v);
 }

 /**
  * idr_get_new_above - allocate new idr entry above or equal to a start id
  * @idp: idr handle
  * @ptr: pointer you want associated with the ide
  * @start_id: id to start search at
  * @id: pointer to the allocated handle
  *
  * This is the allocate id function.  It should be called with any
  * required locks.
  *
  * If memory is required, it will return -EAGAIN, you should unlock
  * and go back to the idr_pre_get() call.  If the idr is full, it will
  * return -ENOSPC.
  *
  * @id returns a value in the range 0 ... 0x7fffffff
  */
 int idr_get_new_above(struct idr *idp, void *ptr, int starting_id, int *id)
 {
 	int rv;

 	rv = idr_get_new_above_int(idp, ptr, starting_id);
 	/*
 	 * This is a cheap hack until the IDR code can be fixed to
 	 * return proper error values.
 	 */
 	if (rv < 0) {
 		if (rv == -1)
 			return -EAGAIN;
 		else /* Will be -3 */
 			return -ENOSPC;
 	}
 	*id = rv;
 	return 0;
 }
 EXPORT_SYMBOL(idr_get_new_above);

 /**
  * idr_get_new - allocate new idr entry
  * @idp: idr handle
  * @ptr: pointer you want associated with the ide
  * @id: pointer to the allocated handle
  *
  * This is the allocate id function.  It should be called with any
  * required locks.
  *
  * If memory is required, it will return -EAGAIN, you should unlock
  * and go back to the idr_pre_get() call.  If the idr is full, it will
  * return -ENOSPC.
  *
  * @id returns a value in the range 0 ... 0x7fffffff
  */
 int idr_get_new(struct idr *idp, void *ptr, int *id)
 {
 	int rv;

 	rv = idr_get_new_above_int(idp, ptr, 0);
 	/*
 	 * This is a cheap hack until the IDR code can be fixed to
 	 * return proper error values.
 	 */
 	if (rv < 0) {
 		if (rv == -1)
 			return -EAGAIN;
 		else /* Will be -3 */
 			return -ENOSPC;
 	}
 	*id = rv;
 	return 0;
 }
 EXPORT_SYMBOL(idr_get_new);

 static void idr_remove_warning(int id)
 {
 	printk("idr_remove called for id=%d which is not allocated.\n", id);
 	dump_stack();
 }

 static void sub_remove(struct idr *idp, int shift, int id)
 {
 	struct idr_layer *p = idp->top;
 	struct idr_layer **pa[MAX_LEVEL];
 	struct idr_layer ***paa = &pa[0];
 	int n;

 	*paa = NULL;
 	*++paa = &idp->top;

 	while ((shift > 0) && p) {
 		n = (id >> shift) & IDR_MASK;
 		__clear_bit(n, &p->bitmap);
 		*++paa = &p->ary[n];
 		p = p->ary[n];
 		shift -= IDR_BITS;
 	}
 	n = id & IDR_MASK;
 	if (likely(p != NULL && test_bit(n, &p->bitmap))){
 		__clear_bit(n, &p->bitmap);
 		p->ary[n] = NULL;
 		while(*paa && ! --((**paa)->count)){
 			free_layer(idp, **paa);
 			**paa-- = NULL;
 		}
 		if (!*paa)
 			idp->layers = 0;
 	} else
 		idr_remove_warning(id);
 }

 /**
  * idr_remove - remove the given id and free it's slot
  * @idp: idr handle
  * @id: unique key
  */
 void idr_remove(struct idr *idp, int id)
 {
 	struct idr_layer *p;

 	/* Mask off upper bits we don't use for the search. */
 	id &= MAX_ID_MASK;

 	sub_remove(idp, (idp->layers - 1) * IDR_BITS, id);
 	if (idp->top && idp->top->count == 1 && (idp->layers > 1) &&
 	    idp->top->ary[0]) {  // We can drop a layer

 		p = idp->top->ary[0];
 		idp->top->bitmap = idp->top->count = 0;
 		free_layer(idp, idp->top);
 		idp->top = p;
 		--idp->layers;
 	}
 	while (idp->id_free_cnt >= IDR_FREE_MAX) {
 		p = alloc_layer(idp);
 		kmem_cache_free(idr_layer_cache, p);
 		return;
 	}
 }
 EXPORT_SYMBOL(idr_remove);

 /**
  * idr_destroy - release all cached layers within an idr tree
  * idp: idr handle
  */
 void idr_destroy(struct idr *idp)
 {
 	while (idp->id_free_cnt) {
 		struct idr_layer *p = alloc_layer(idp);
 		kmem_cache_free(idr_layer_cache, p);
 	}
 }
 EXPORT_SYMBOL(idr_destroy);

 /**
  * idr_find - return pointer for given id
  * @idp: idr handle
  * @id: lookup key
  *
  * Return the pointer given the id it has been registered with.  A %NULL
  * return indicates that @id is not valid or you passed %NULL in
  * idr_get_new().
  *
  * The caller must serialize idr_find() vs idr_get_new() and idr_remove().
  */
 void *idr_find(struct idr *idp, int id)
 {
 	int n;
 	struct idr_layer *p;

 	n = idp->layers * IDR_BITS;
 	p = idp->top;

 	/* Mask off upper bits we don't use for the search. */
 	id &= MAX_ID_MASK;

 	if (id >= (1 << n))
 		return NULL;

 	while (n > 0 && p) {
 		n -= IDR_BITS;
 		p = p->ary[(id >> n) & IDR_MASK];
 	}
 	return((void *)p);
 }
 EXPORT_SYMBOL(idr_find);

 /**
  * idr_replace - replace pointer for given id
  * @idp: idr handle
  * @ptr: pointer you want associated with the id
  * @id: lookup key
  *
  * Replace the pointer registered with an id and return the old value.
  * A -ENOENT return indicates that @id was not found.
  * A -EINVAL return indicates that @id was not within valid constraints.
  *
  * The caller must serialize vs idr_find(), idr_get_new(), and idr_remove().
  */
 void *idr_replace(struct idr *idp, void *ptr, int id)
 {
 	int n;
 	struct idr_layer *p, *old_p;

 	n = idp->layers * IDR_BITS;
 	p = idp->top;

 	id &= MAX_ID_MASK;

 	if (id >= (1 << n))
 		return ERR_PTR(-EINVAL);

 	n -= IDR_BITS;
 	while ((n > 0) && p) {
 		p = p->ary[(id >> n) & IDR_MASK];
 		n -= IDR_BITS;
 	}

 	n = id & IDR_MASK;
 	if (unlikely(p == NULL || !test_bit(n, &p->bitmap)))
 		return ERR_PTR(-ENOENT);

 	old_p = p->ary[n];
 	p->ary[n] = ptr;

 	return old_p;
 }
 EXPORT_SYMBOL(idr_replace);

 static void idr_cache_ctor(void * idr_layer, struct kmem_cache *idr_layer_cache,
 		unsigned long flags)
 {
 	memset(idr_layer, 0, sizeof(struct idr_layer));
 }

 static  int init_id_cache(void)
 {
 	if (!idr_layer_cache)
 		idr_layer_cache = kmem_cache_create("idr_layer_cache",
 			sizeof(struct idr_layer), 0, 0, idr_cache_ctor, NULL);
 	return 0;
 }

 /**
  * idr_init - initialize idr handle
  * @idp:	idr handle
  *
  * This function is use to set up the handle (@idp) that you will pass
  * to the rest of the functions.
  */
 void idr_init(struct idr *idp)
 {
 	init_id_cache();
 	memset(idp, 0, sizeof(struct idr));
 	spin_lock_init(&idp->lock);
 }
 EXPORT_SYMBOL(idr_init);
	/*
	* 2002-10-18 written by Jim Houston jim.houston@ccur.com
	* Copyright (C) 2002 by Concurrent Computer Corporation
	* Distributed under the GNU GPL license version 2.
	*
	* Modified by George Anzinger to reuse immediately and to use
	* find bit instructions. Also removed _irq on spinlocks.
	*
	* Small id to pointer translation service.
	*
	* It uses a radix tree like structure as a sparse array indexed
	* by the id to obtain the pointer. The bitmap makes allocating
	* a new id quick.
	*
	* You call it to allocate an id (an int) an associate with that id a
	* pointer or what ever, we treat it as a (void *). You can pass this
	* id to a user for him to pass back at a later time. You then pass
	* that id to this code and it returns your pointer.

	* You can release ids at any time. When all ids are released, most of
	* the memory is returned (we keep IDR_FREE_MAX) in a local pool so we
	* don't need to go to the memory "store" during an id allocate, just
	* so you don't need to be too concerned about locking and conflicts
	* with the slab allocator.
	*/

	#ifndef TEST // to test in user space...
	#include <linux/slab.h>
	#include <linux/init.h>
	#include <linux/module.h>
	#endif
	#include <linux/err.h>
	#include <linux/string.h>
	#include <linux/idr.h>

	static struct kmem_cache *idr_layer_cache;

	static struct idr_layer alloc_layer(struct idr idp)
	{
	struct idr_layer *p;
	unsigned long flags;

	spin_lock_irqsave(&idp->lock, flags);
	if ((p = idp->id_free)) {
	idp->id_free = p->ary[0];
	idp->id_free_cnt--;
	p->ary[0] = NULL;
	}
	spin_unlock_irqrestore(&idp->lock, flags);
	return(p);
	}

	/* only called when idp->lock is held */
	static void __free_layer(struct idr idp, struct idr_layer p)
	{
	p->ary[0] = idp->id_free;
	idp->id_free = p;
	idp->id_free_cnt++;
	}

	static void free_layer(struct idr idp, struct idr_layer p)
	{
	unsigned long flags;

	/*
	* Depends on the return element being zeroed.
	*/
	spin_lock_irqsave(&idp->lock, flags);
	__free_layer(idp, p);
	spin_unlock_irqrestore(&idp->lock, flags);
	}

	/**
	* idr_pre_get - reserver resources for idr allocation
	* @idp: idr handle
	* @gfp_mask: memory allocation flags
	*
	* This function should be called prior to locking and calling the
	* following function. It preallocates enough memory to satisfy
	* the worst possible allocation.
	*
	* If the system is REALLY out of memory this function returns 0,
	* otherwise 1.
	*/
	int idr_pre_get(struct idr *idp, gfp_t gfp_mask)
	{
	while (idp->id_free_cnt < IDR_FREE_MAX) {
	struct idr_layer *new;
	new = kmem_cache_alloc(idr_layer_cache, gfp_mask);
	if (new == NULL)
	return (0);
	free_layer(idp, new);
	}
	return 1;
	}
	EXPORT_SYMBOL(idr_pre_get);

	static int sub_alloc(struct idr idp, void ptr, int *starting_id)
	{
	int n, m, sh;
	struct idr_layer p, new;
	struct idr_layer *pa[MAX_LEVEL];
	int l, id, oid;
	long bm;

	id = *starting_id;
	restart:
	p = idp->top;
	l = idp->layers;
	pa[l--] = NULL;
	while (1) {
	/*
	* We run around this while until we reach the leaf node...
	*/
	n = (id >> (IDR_BITS*l)) & IDR_MASK;
	bm = ~p->bitmap;
	m = find_next_bit(&bm, IDR_SIZE, n);
	if (m == IDR_SIZE) {
	/* no space available go back to previous layer. */
	l++;
	oid = id;
	id = (id \| ((1 << (IDR_BITS * l)) - 1)) + 1;

	/* if already at the top layer, we need to grow */
	if (!(p = pa[l])) {
	*starting_id = id;
	return -2;
	}

	/* If we need to go up one layer, continue the
	* loop; otherwise, restart from the top.
	*/
	sh = IDR_BITS * (l + 1);
	if (oid >> sh == id >> sh)
	continue;
	else
	goto restart;
	}
	if (m != n) {
	sh = IDR_BITS*l;
	id = ((id >> sh) ^ n ^ m) << sh;
	}
	if ((id >= MAX_ID_BIT) \|\| (id < 0))
	return -3;
	if (l == 0)
	break;
	/*
	* Create the layer below if it is missing.
	*/
	if (!p->ary[m]) {
	if (!(new = alloc_layer(idp)))
	return -1;
	p->ary[m] = new;
	p->count++;
	}
	pa[l--] = p;
	p = p->ary[m];
	}
	/*
	* We have reached the leaf node, plant the
	* users pointer and return the raw id.
	*/
	p->ary[m] = (struct idr_layer *)ptr;
	__set_bit(m, &p->bitmap);
	p->count++;
	/*
	* If this layer is full mark the bit in the layer above
	* to show that this part of the radix tree is full.
	* This may complete the layer above and require walking
	* up the radix tree.
	*/
	n = id;
	while (p->bitmap == IDR_FULL) {
	if (!(p = pa[++l]))
	break;
	n = n >> IDR_BITS;
	__set_bit((n & IDR_MASK), &p->bitmap);
	}
	return(id);
	}

	static int idr_get_new_above_int(struct idr idp, void ptr, int starting_id)
	{
	struct idr_layer p, new;
	int layers, v, id;
	unsigned long flags;

	id = starting_id;
	build_up:
	p = idp->top;
	layers = idp->layers;
	if (unlikely(!p)) {
	if (!(p = alloc_layer(idp)))
	return -1;
	layers = 1;
	}
	/*
	* Add a new layer to the top of the tree if the requested
	* id is larger than the currently allocated space.
	*/
	while ((layers < (MAX_LEVEL - 1)) && (id >= (1 << (layers*IDR_BITS)))) {
	layers++;
	if (!p->count)
	continue;
	if (!(new = alloc_layer(idp))) {
	/*
	* The allocation failed. If we built part of
	* the structure tear it down.
	*/
	spin_lock_irqsave(&idp->lock, flags);
	for (new = p; p && p != idp->top; new = p) {
	p = p->ary[0];
	new->ary[0] = NULL;
	new->bitmap = new->count = 0;
	__free_layer(idp, new);
	}
	spin_unlock_irqrestore(&idp->lock, flags);
	return -1;
	}
	new->ary[0] = p;
	new->count = 1;
	if (p->bitmap == IDR_FULL)
	__set_bit(0, &new->bitmap);
	p = new;
	}
	idp->top = p;
	idp->layers = layers;
	v = sub_alloc(idp, ptr, &id);
	if (v == -2)
	goto build_up;
	return(v);
	}

	/**
	* idr_get_new_above - allocate new idr entry above or equal to a start id
	* @idp: idr handle
	* @ptr: pointer you want associated with the ide
	* @start_id: id to start search at
	* @id: pointer to the allocated handle
	*
	* This is the allocate id function. It should be called with any
	* required locks.
	*
	* If memory is required, it will return -EAGAIN, you should unlock
	* and go back to the idr_pre_get() call. If the idr is full, it will
	* return -ENOSPC.
	*
	* @id returns a value in the range 0 ... 0x7fffffff
	*/
	int idr_get_new_above(struct idr idp, void ptr, int starting_id, int *id)
	{
	int rv;

	rv = idr_get_new_above_int(idp, ptr, starting_id);
	/*
	* This is a cheap hack until the IDR code can be fixed to
	* return proper error values.
	*/
	if (rv < 0) {
	if (rv == -1)
	return -EAGAIN;
	else /* Will be -3 */
	return -ENOSPC;
	}
	*id = rv;
	return 0;
	}
	EXPORT_SYMBOL(idr_get_new_above);

	/**
	* idr_get_new - allocate new idr entry
	* @idp: idr handle
	* @ptr: pointer you want associated with the ide
	* @id: pointer to the allocated handle
	*
	* This is the allocate id function. It should be called with any
	* required locks.
	*
	* If memory is required, it will return -EAGAIN, you should unlock
	* and go back to the idr_pre_get() call. If the idr is full, it will
	* return -ENOSPC.
	*
	* @id returns a value in the range 0 ... 0x7fffffff
	*/
	int idr_get_new(struct idr idp, void ptr, int *id)
	{
	int rv;

	rv = idr_get_new_above_int(idp, ptr, 0);
	/*
	* This is a cheap hack until the IDR code can be fixed to
	* return proper error values.
	*/
	if (rv < 0) {
	if (rv == -1)
	return -EAGAIN;
	else /* Will be -3 */
	return -ENOSPC;
	}
	*id = rv;
	return 0;
	}
	EXPORT_SYMBOL(idr_get_new);

	static void idr_remove_warning(int id)
	{
	printk("idr_remove called for id=%d which is not allocated.\n", id);
	dump_stack();
	}

	static void sub_remove(struct idr *idp, int shift, int id)
	{
	struct idr_layer *p = idp->top;
	struct idr_layer **pa[MAX_LEVEL];
	struct idr_layer ***paa = &pa[0];
	int n;

	*paa = NULL;
	*++paa = &idp->top;

	while ((shift > 0) && p) {
	n = (id >> shift) & IDR_MASK;
	__clear_bit(n, &p->bitmap);
	*++paa = &p->ary[n];
	p = p->ary[n];
	shift -= IDR_BITS;
	}
	n = id & IDR_MASK;
	if (likely(p != NULL && test_bit(n, &p->bitmap))){
	__clear_bit(n, &p->bitmap);
	p->ary[n] = NULL;
	while(paa && ! --((*paa)->count)){
	free_layer(idp, **paa);
	**paa-- = NULL;
	}
	if (!*paa)
	idp->layers = 0;
	} else
	idr_remove_warning(id);
	}

	/**
	* idr_remove - remove the given id and free it's slot
	* @idp: idr handle
	* @id: unique key
	*/
	void idr_remove(struct idr *idp, int id)
	{
	struct idr_layer *p;

	/* Mask off upper bits we don't use for the search. */
	id &= MAX_ID_MASK;

	sub_remove(idp, (idp->layers - 1) * IDR_BITS, id);
	if (idp->top && idp->top->count == 1 && (idp->layers > 1) &&
	idp->top->ary[0]) { // We can drop a layer

	p = idp->top->ary[0];
	idp->top->bitmap = idp->top->count = 0;
	free_layer(idp, idp->top);
	idp->top = p;
	--idp->layers;
	}
	while (idp->id_free_cnt >= IDR_FREE_MAX) {
	p = alloc_layer(idp);
	kmem_cache_free(idr_layer_cache, p);
	return;
	}
	}
	EXPORT_SYMBOL(idr_remove);

	/**
	* idr_destroy - release all cached layers within an idr tree
	* idp: idr handle
	*/
	void idr_destroy(struct idr *idp)
	{
	while (idp->id_free_cnt) {
	struct idr_layer *p = alloc_layer(idp);
	kmem_cache_free(idr_layer_cache, p);
	}
	}
	EXPORT_SYMBOL(idr_destroy);

	/**
	* idr_find - return pointer for given id
	* @idp: idr handle
	* @id: lookup key
	*
	* Return the pointer given the id it has been registered with. A %NULL
	* return indicates that @id is not valid or you passed %NULL in
	* idr_get_new().
	*
	* The caller must serialize idr_find() vs idr_get_new() and idr_remove().
	*/
	void idr_find(struct idr idp, int id)
	{
	int n;
	struct idr_layer *p;

	n = idp->layers * IDR_BITS;
	p = idp->top;

	/* Mask off upper bits we don't use for the search. */
	id &= MAX_ID_MASK;

	if (id >= (1 << n))
	return NULL;

	while (n > 0 && p) {
	n -= IDR_BITS;
	p = p->ary[(id >> n) & IDR_MASK];
	}
	return((void *)p);
	}
	EXPORT_SYMBOL(idr_find);

	/**
	* idr_replace - replace pointer for given id
	* @idp: idr handle
	* @ptr: pointer you want associated with the id
	* @id: lookup key
	*
	* Replace the pointer registered with an id and return the old value.
	* A -ENOENT return indicates that @id was not found.
	* A -EINVAL return indicates that @id was not within valid constraints.
	*
	* The caller must serialize vs idr_find(), idr_get_new(), and idr_remove().
	*/
	void idr_replace(struct idr idp, void *ptr, int id)
	{
	int n;
	struct idr_layer p, old_p;

	n = idp->layers * IDR_BITS;
	p = idp->top;

	id &= MAX_ID_MASK;

	if (id >= (1 << n))
	return ERR_PTR(-EINVAL);

	n -= IDR_BITS;
	while ((n > 0) && p) {
	p = p->ary[(id >> n) & IDR_MASK];
	n -= IDR_BITS;
	}

	n = id & IDR_MASK;
	if (unlikely(p == NULL \|\| !test_bit(n, &p->bitmap)))
	return ERR_PTR(-ENOENT);

	old_p = p->ary[n];
	p->ary[n] = ptr;

	return old_p;
	}
	EXPORT_SYMBOL(idr_replace);

	static void idr_cache_ctor(void * idr_layer, struct kmem_cache *idr_layer_cache,
	unsigned long flags)
	{
	memset(idr_layer, 0, sizeof(struct idr_layer));
	}

	static int init_id_cache(void)
	{
	if (!idr_layer_cache)
	idr_layer_cache = kmem_cache_create("idr_layer_cache",
	sizeof(struct idr_layer), 0, 0, idr_cache_ctor, NULL);
	return 0;
	}

	/**
	* idr_init - initialize idr handle
	* @idp: idr handle
	*
	* This function is use to set up the handle (@idp) that you will pass
	* to the rest of the functions.
	*/
	void idr_init(struct idr *idp)
	{
	init_id_cache();
	memset(idp, 0, sizeof(struct idr));
	spin_lock_init(&idp->lock);
	}
	EXPORT_SYMBOL(idr_init);