root/lib/idr.c

/*
 * linux/kernel/id.c
 *
 * 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.
 *
 * 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.  

 * Modified by George Anzinger to reuse immediately and to use
 * find bit instructions.  Also removed _irq on spinlocks.

 * So here is what this bit of code does:

 * 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.

 * A word on reuse.  We reuse empty id slots as soon as we can, always
 * using the lowest one available.  But we also merge a counter in the
 * high bits of the id.  The counter is RESERVED_ID_BITS (8 at this time)
 * long.  This means that if you allocate and release the same id in a 
 * loop we will reuse an id after about 256 times around the loop.  The
 * word about is used here as we will NOT return a valid id of -1 so if
 * you loop on the largest possible id (and that is 24 bits, wow!) we
 * will kick the counter to avoid -1.  (Paranoid?  You bet!)
 *
 * What you need to do is, since we don't keep the counter as part of
 * id / ptr pair, to keep a copy of it in the pointed to structure
 * (or else where) so that when you ask for a ptr you can varify that
 * the returned ptr is correct by comparing the id it contains with the one
 * you asked for.  In other words, we only did half the reuse protection.
 * Since the code depends on your code doing this check, we ignore high
 * order bits in the id, not just the count, but bits that would, if used,
 * index outside of the allocated ids.  In other words, if the largest id
 * currently allocated is 32 a look up will only look at the low 5 bits of
 * the id.  Since you will want to keep this id in the structure anyway
 * (if for no other reason than to be able to eliminate the id when the
 * structure is found in some other way) this seems reasonable.  If you
 * really think otherwise, the code to check these bits here, it is just
 * disabled with a #if 0.


 * So here are the complete details:

 *  include <linux/idr.h>

 * void idr_init(struct idr *idp)

 *   This function is use to set up the handle (idp) that you will pass
 *   to the rest of the functions.  The structure is defined in the
 *   header.

 * int idr_pre_get(struct idr *idp, unsigned gfp_mask)

 *   This function should be called prior to locking and calling the
 *   following function.  It pre allocates enough memory to satisfy the
 *   worst possible allocation.  Unless gfp_mask is GFP_ATOMIC, it can
 *   sleep, so must not be called with any spinlocks held.  If the system is
 *   REALLY out of memory this function returns 0, other wise 1.

 * int idr_get_new(struct idr *idp, void *ptr);
 
 *   This is the allocate id function.  It should be called with any
 *   required locks.  In fact, in the SMP case, you MUST lock prior to
 *   calling this function to avoid possible out of memory problems.  If
 *   memory is required, it will return a -1, in which case you should
 *   unlock and go back to the idr_pre_get() call.  ptr is the pointer
 *   you want associated with the id.  In other words:

 * void *idr_find(struct idr *idp, int id);
 
 *   returns the "ptr", given the id.  A NULL return indicates that the
 *   id is not valid (or you passed NULL in the idr_get_new(), shame on
 *   you).  This function must be called with a spinlock that prevents
 *   calling either idr_get_new() or idr_remove() or idr_find() while it
 *   is working.

 * void idr_remove(struct idr *idp, int id);

 *   removes the given id, freeing that slot and any memory that may
 *   now be unused.  See idr_find() for locking restrictions.

 */



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


static kmem_cache_t *idr_layer_cache;



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

        spin_lock(&idp->lock);
        if (!(p = idp->id_free))
                BUG();
        idp->id_free = p->ary[0];
        idp->id_free_cnt--;
        p->ary[0] = 0;
        spin_unlock(&idp->lock);
        return(p);
}

static void free_layer(struct idr *idp, struct idr_layer *p)
{
        /*
         * Depends on the return element being zeroed.
         */
        spin_lock(&idp->lock);
        p->ary[0] = idp->id_free;
        idp->id_free = p;
        idp->id_free_cnt++;
        spin_unlock(&idp->lock);
}

int idr_pre_get(struct idr *idp, unsigned 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;
        long bm;

        id = *starting_id;
        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++;
                        id = (id | ((1 << (IDR_BITS*l))-1)) + 1;
                        if (!(p = pa[l])) {
                                *starting_id = id;
                                return -2;
                        }
                        continue;
                }
                if (m != n) {
                        sh = IDR_BITS*l;
                        id = ((id >> sh) ^ n ^ m) << sh;
                }
                if (id >= MAX_ID_BIT)
                        return -1;
                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);
}

int idr_get_new_above(struct idr *idp, void *ptr, int starting_id)
{
        struct idr_layer *p, *new;
        int layers, v, id;
        
        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 (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.
                         */
                        for (new = p; p && p != idp->top; new = p) {
                                p = p->ary[0];
                                new->ary[0] = 0;
                                new->bitmap = new->count = 0;
                                free_layer(idp, new);
                        }
                        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;
        if ( likely(v >= 0 )) {
                idp->count++;
                v += (idp->count << MAX_ID_SHIFT);
                if ( unlikely( v == -1 ))
                     v += (1L << MAX_ID_SHIFT);
        }
        return(v);
}
EXPORT_SYMBOL(idr_get_new_above);

int idr_get_new(struct idr *idp, void *ptr)
{
        return idr_get_new_above(idp, ptr, 0);
}
EXPORT_SYMBOL(idr_get_new);


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];

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

        while ((shift > 0) && p) {
                int n = (id >> shift) & IDR_MASK;
                __clear_bit(n, &p->bitmap);
                *++paa = &p->ary[n];
                p = p->ary[n];
                shift -= IDR_BITS;
        }
        if (likely(p != NULL)){
                int n = id & IDR_MASK;
                __clear_bit(n, &p->bitmap);
                p->ary[n] = NULL;
                while(*paa && ! --((**paa)->count)){
                        free_layer(idp, **paa);
                        **paa-- = NULL;
                }
                if ( ! *paa )
                        idp->layers = 0;
        }
}
void idr_remove(struct idr *idp, int id)
{
        struct idr_layer *p;

        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);

void *idr_find(struct idr *idp, int id)
{
        int n;
        struct idr_layer *p;

        n = idp->layers * IDR_BITS;
        p = idp->top;
#if 0
        /*
         * This tests to see if bits outside the current tree are
         * present.  If so, tain't one of ours!
         */
        if ( unlikely( (id & ~(~0 << MAX_ID_SHIFT)) >> (n + IDR_BITS)))
             return NULL;
#endif
        while (n > 0 && p) {
                n -= IDR_BITS;
                p = p->ary[(id >> n) & IDR_MASK];
        }
        return((void *)p);
}
EXPORT_SYMBOL(idr_find);

static void idr_cache_ctor(void * idr_layer, 
                           kmem_cache_t *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, 0);
        return 0;
}

void idr_init(struct idr *idp)
{
        init_id_cache();
        memset(idp, 0, sizeof(struct idr));
        spin_lock_init(&idp->lock);
}
EXPORT_SYMBOL(idr_init);