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    SubjectRe: [PATCH 0/9] Scalability requirements for sysv ipc - v3
    On Wed, May 07, 2008 at 01:35:53PM +0200, Nadia.Derbey@bull.net wrote:
    >
    > After scalability problems have been detected when using the sysV ipcs, I
    > have proposed to use an RCU based implementation of the IDR api instead (see
    > threads http://lkml.org/lkml/2008/4/11/212 and
    > http://lkml.org/lkml/2008/4/29/295).
    >
    > This resulted in many people asking to convert the idr API and make it
    > rcu safe (because most of the code was duplicated and thus unmaintanable
    > and unreviewable).
    >
    > So here is a first attempt.
    >
    > The important change wrt to the idr API itself is during idr removes:
    > idr layers are freed after a grace period, instead of being moved to the
    > free list.
    >
    > The important change wrt to ipcs, is that idr_find() can now be called
    > locklessly inside a rcu read critical section.
    >
    > Here are the results I've got for the pmsg test sent by Manfred:
    >
    > 2.6.25-rc3-mm1 2.6.25-rc3-mm1+ 2.6.25-mm1 Patched 2.6.25-mm1
    > 1 1168441 1064021 876000 947488
    > 2 1094264 921059 1549592 1730685
    > 3 2082520 1738165 1694370 2324880
    > 4 2079929 1695521 404553 2400408
    > 5 2898758 406566 391283 3246580
    > 6 2921417 261275 263249 3752148
    > 7 3308761 126056 191742 4243142
    > 8 3329456 100129 141722 4275780
    >
    > 1st column: stock 2.6.25-rc3-mm1
    > 2nd column: 2.6.25-rc3-mm1 + ipc patches (store ipcs into idrs)
    > 3nd column: stock 2.6.25-mm1
    > 4th column: 2.6.25-mm1 + this pacth series.
    >
    > I'll send a chart as an answer to this mail: don't know how to do that
    > with quilt :-(
    >
    >
    > Reviewers are more than ever welcome!
    >
    > Patches should be applied on linux-2.6.25-mm1, in the following order:
    >
    > [ PATCH 01/09 ] : idr_add_rcu_head.patch
    > [ PATCH 02/09 ] : idr_rename_routines.patch
    > [ PATCH 03/09 ] : idr_fix_printk.patch
    > [ PATCH 04/09 ] : idr_rc_to_errno.patch
    > [ PATCH 05/09 ] : idr_get_new_rcu_safe.patch
    > [ PATCH 06/09 ] : idr_find_rcu_safe.patch
    > [ PATCH 07/09 ] : idr_remove_rcu_safe.patch
    > [ PATCH 08/09 ] : ipc_fix_ipc_lock.patch
    > [ PATCH 09/09 ] : remove_ipc_lock_down.patch
    >
    > Patches 2, 3 and 4 do not introduce actual changes.
    >
    > I won't be available before next Tuesday, so, please, don't be mad at me if
    > I'm not answering fast enough.

    I guess in my case, next Tuesday was not an issue. :-/

    Anyway, the idr.c changes look good to me. Not sure why you are using
    INIT_RCU_HEAD() given that call_rcu() completely initializes the fields.
    Using INIT_RCU_HEAD() doesn't cause any problems, but does add needless
    code.

    Commentary below, looks good from an RCU viewpoint.

    Thanx, Paul

    > /*
    > * 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.
    > *
    > * Modified by Nadia Derbey to make it RCU safe.
    > *
    > * 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 *get_from_free_list(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;

    OK, this is the freelist which is inaccessible to readers.

    > }
    > spin_unlock_irqrestore(&idp->lock, flags);
    > return(p);
    > }
    >
    > static void idr_layer_rcu_free(struct rcu_head *head)
    > {
    > struct idr_layer *layer;
    >
    > layer = container_of(head, struct idr_layer, rcu_head);
    > kmem_cache_free(idr_layer_cache, layer);
    > }
    >
    > static inline void free_layer(struct idr_layer *p)
    > {
    > call_rcu(&p->rcu_head, idr_layer_rcu_free);
    > }
    >
    > /* only called when idp->lock is held */
    > static void __move_to_free_list(struct idr *idp, struct idr_layer *p)
    > {
    > p->ary[0] = idp->id_free;

    OK, this is the freelist which is inaccessible to readers.

    > idp->id_free = p;
    > idp->id_free_cnt++;
    > }
    >
    > static void move_to_free_list(struct idr *idp, struct idr_layer *p)
    > {
    > unsigned long flags;
    >
    > /*
    > * Depends on the return element being zeroed.
    > */
    > spin_lock_irqsave(&idp->lock, flags);
    > __move_to_free_list(idp, p);
    > spin_unlock_irqrestore(&idp->lock, flags);
    > }
    >
    > static void idr_mark_full(struct idr_layer **pa, int id)
    > {
    > struct idr_layer *p = pa[0];
    > int l = 0;
    >
    > __set_bit(id & IDR_MASK, &p->bitmap);
    > /*
    > * 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.
    > */
    > while (p->bitmap == IDR_FULL) {
    > if (!(p = pa[++l]))
    > break;
    > id = id >> IDR_BITS;
    > __set_bit((id & IDR_MASK), &p->bitmap);
    > }
    > }
    >
    > /**
    > * 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
    > * idr_get_new* functions. 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);
    > move_to_free_list(idp, new);
    > }
    > return 1;
    > }
    > EXPORT_SYMBOL(idr_pre_get);
    >
    > static int sub_alloc(struct idr *idp, int *starting_id, struct idr_layer **pa)
    > {
    > int n, m, sh;
    > struct idr_layer *p, *new;
    > int l, id, oid;
    > unsigned long bm;
    >
    > id = *starting_id;
    > restart:
    > p = idp->top;

    OK, the caller presumably holds an update-side lock.

    > 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 IDR_NEED_TO_GROW;
    > }
    >
    > /* 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 IDR_NOMORE_SPACE;
    > if (l == 0)
    > break;
    > /*
    > * Create the layer below if it is missing.
    > */
    > if (!p->ary[m]) {

    OK, we aren't dereferencing. Besides, we should hold the update-side
    lock at this point.

    > new = get_from_free_list(idp);
    > if (!new)
    > return -1;
    > INIT_RCU_HEAD(&new->rcu_head);

    Not needed, unless you want this zeroed for debug purposes.

    > rcu_assign_pointer(p->ary[m], new);
    > p->count++;
    > }
    > pa[l--] = p;
    > p = p->ary[m];

    Holding update-side lock.

    > }
    >
    > pa[l] = p;
    > return id;
    > }
    >
    > static int idr_get_empty_slot(struct idr *idp, int starting_id,
    > struct idr_layer **pa)
    > {
    > struct idr_layer *p, *new;
    > int layers, v, id;
    > unsigned long flags;
    >
    > id = starting_id;
    > build_up:
    > p = idp->top;

    OK, the caller presumably holds an update-side lock.

    > layers = idp->layers;
    > if (unlikely(!p)) {
    > if (!(p = get_from_free_list(idp)))
    > return -1;
    > INIT_RCU_HEAD(&p->rcu_head);

    Not needed, unless you want this zeroed for debug purposes.

    > 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 = get_from_free_list(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;

    OK, this presumably has not yet been made accessible to readers.

    > new->bitmap = new->count = 0;
    > __move_to_free_list(idp, new);
    > }
    > spin_unlock_irqrestore(&idp->lock, flags);
    > return -1;
    > }
    > new->ary[0] = p;

    OK, this presumably has not yet been made accessible to readers.

    > new->count = 1;
    > INIT_RCU_HEAD(&new->rcu_head);

    Not needed, unless you want this zeroed for debug purposes.

    > if (p->bitmap == IDR_FULL)
    > __set_bit(0, &new->bitmap);
    > p = new;
    > }
    > rcu_assign_pointer(idp->top, p);
    > idp->layers = layers;
    > v = sub_alloc(idp, &id, pa);
    > if (v == IDR_NEED_TO_GROW)
    > goto build_up;
    > return(v);
    > }
    >
    > static int idr_get_new_above_int(struct idr *idp, void *ptr, int starting_id)
    > {
    > struct idr_layer *pa[MAX_LEVEL];
    > int id;
    >
    > id = idr_get_empty_slot(idp, starting_id, pa);
    > if (id >= 0) {
    > /*
    > * Successfully found an empty slot. Install the user
    > * pointer and mark the slot full.
    > */
    > rcu_assign_pointer(pa[0]->ary[id & IDR_MASK],
    > (struct idr_layer *)ptr);
    > pa[0]->count++;
    > idr_mark_full(pa, id);
    > }
    >
    > return id;
    > }
    >
    > /**
    > * 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)
    > return _idr_rc_to_errno(rv);
    > *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)
    > return _idr_rc_to_errno(rv);
    > *id = rv;
    > return 0;
    > }
    > EXPORT_SYMBOL(idr_get_new);
    >
    > static void idr_remove_warning(int id)
    > {
    > printk(KERN_WARNING
    > "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;

    OK, the caller presumably holds an update-side lock.

    > struct idr_layer **pa[MAX_LEVEL];
    > struct idr_layer ***paa = &pa[0];
    > struct idr_layer *to_free;
    > 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];

    OK, the caller presumably holds an update-side lock.

    > 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);
    > rcu_assign_pointer(p->ary[n], NULL);
    > to_free = NULL;
    > while(*paa && ! --((**paa)->count)){
    > if (to_free)
    > free_layer(to_free);
    > to_free = **paa;
    > **paa-- = NULL;
    > }
    > if (!*paa)
    > idp->layers = 0;
    > if (to_free)
    > free_layer(to_free);
    > } 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;
    > struct idr_layer *to_free;
    >
    > /* 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]) {

    OK, the caller presumably holds the update-side lock.

    > /*
    > * Single child at leftmost slot: we can shrink the tree.
    > * This level is not needed anymore since when layers are
    > * inserted, they are inserted at the top of the existing
    > * tree.
    > */
    > to_free = idp->top;
    > p = idp->top->ary[0];

    OK, the caller presumably holds the update-side lock.

    > rcu_assign_pointer(idp->top, p);
    > --idp->layers;
    > to_free->bitmap = to_free->count = 0;
    > free_layer(to_free);
    > }
    > while (idp->id_free_cnt >= IDR_FREE_MAX) {
    > p = get_from_free_list(idp);
    > /*
    > * Note: we don't call the rcu callback here, since the only
    > * layers that fall into the freelist are those that have been
    > * preallocated.
    > */
    > kmem_cache_free(idr_layer_cache, p);
    > }
    > return;
    > }
    > EXPORT_SYMBOL(idr_remove);
    >
    > /**
    > * idr_remove_all - remove all ids from the given idr tree
    > * @idp: idr handle
    > *
    > * idr_destroy() only frees up unused, cached idp_layers, but this
    > * function will remove all id mappings and leave all idp_layers
    > * unused.
    > *
    > * A typical clean-up sequence for objects stored in an idr tree, will
    > * use idr_for_each() to free all objects, if necessay, then
    > * idr_remove_all() to remove all ids, and idr_destroy() to free
    > * up the cached idr_layers.
    > */
    > void idr_remove_all(struct idr *idp)
    > {
    > int n, id, max;
    > struct idr_layer *p;
    > struct idr_layer *pa[MAX_LEVEL];
    > struct idr_layer **paa = &pa[0];
    >
    > n = idp->layers * IDR_BITS;
    > p = idp->top;

    OK, the caller presumably holds an update-side lock.

    > max = 1 << n;
    >
    > id = 0;
    > while (id < max) {
    > while (n > IDR_BITS && p) {
    > n -= IDR_BITS;
    > *paa++ = p;
    > p = p->ary[(id >> n) & IDR_MASK];

    OK, the caller presumably holds the update-side lock.

    > }
    >
    > id += 1 << n;
    > while (n < fls(id)) {
    > if (p)
    > free_layer(p);
    > n += IDR_BITS;
    > p = *--paa;
    > }
    > }
    > rcu_assign_pointer(idp->top, NULL);
    > idp->layers = 0;
    > }
    > EXPORT_SYMBOL(idr_remove_all);
    >
    > /**
    > * 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 = get_from_free_list(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().
    > *
    > * This function can be called under rcu_read_lock(), given that the leaf
    > * pointers lifetimes are correctly managed.
    > */
    > void *idr_find(struct idr *idp, int id)
    > {
    > int n;
    > struct idr_layer *p;
    >
    > n = idp->layers * IDR_BITS;
    > p = rcu_dereference(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 = rcu_dereference(p->ary[(id >> n) & IDR_MASK]);
    > }
    > return((void *)p);
    > }
    > EXPORT_SYMBOL(idr_find);
    >
    > /**
    > * idr_for_each - iterate through all stored pointers
    > * @idp: idr handle
    > * @fn: function to be called for each pointer
    > * @data: data passed back to callback function
    > *
    > * Iterate over the pointers registered with the given idr. The
    > * callback function will be called for each pointer currently
    > * registered, passing the id, the pointer and the data pointer passed
    > * to this function. It is not safe to modify the idr tree while in
    > * the callback, so functions such as idr_get_new and idr_remove are
    > * not allowed.
    > *
    > * We check the return of @fn each time. If it returns anything other
    > * than 0, we break out and return that value.
    > *
    > * The caller must serialize idr_for_each() vs idr_get_new() and idr_remove().
    > */
    > int idr_for_each(struct idr *idp,
    > int (*fn)(int id, void *p, void *data), void *data)
    > {
    > int n, id, max, error = 0;
    > struct idr_layer *p;
    > struct idr_layer *pa[MAX_LEVEL];
    > struct idr_layer **paa = &pa[0];
    >
    > n = idp->layers * IDR_BITS;
    > p = rcu_dereference(idp->top);
    > max = 1 << n;
    >
    > id = 0;
    > while (id < max) {
    > while (n > 0 && p) {
    > n -= IDR_BITS;
    > *paa++ = p;
    > p = rcu_dereference(p->ary[(id >> n) & IDR_MASK]);
    > }
    >
    > if (p) {
    > error = fn(id, (void *)p, data);
    > if (error)
    > break;
    > }
    >
    > id += 1 << n;
    > while (n < fls(id)) {
    > n += IDR_BITS;
    > p = *--paa;
    > }
    > }
    >
    > return error;
    > }
    > EXPORT_SYMBOL(idr_for_each);
    >
    > /**
    > * 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 with writers.
    > */
    > 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;

    OK, the caller presumably holds an update-side lock.

    >
    > 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];

    OK, the caller presumably holds the update-side lock.

    > 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];

    OK, the caller presumably holds the update-side lock.

    > rcu_assign_pointer(p->ary[n], ptr);
    >
    > return old_p;
    > }
    > EXPORT_SYMBOL(idr_replace);
    >
    > static void idr_cache_ctor(struct kmem_cache *idr_layer_cache, void *idr_layer)
    > {
    > memset(idr_layer, 0, sizeof(struct idr_layer));
    > }
    >
    > void __init idr_init_cache(void)
    > {
    > idr_layer_cache = kmem_cache_create("idr_layer_cache",
    > sizeof(struct idr_layer), 0, SLAB_PANIC,
    > idr_cache_ctor);
    > }
    >
    > /**
    > * 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)
    > {
    > memset(idp, 0, sizeof(struct idr));
    > spin_lock_init(&idp->lock);
    > }
    > EXPORT_SYMBOL(idr_init);
    >
    >
    > /*
    > * IDA - IDR based ID allocator
    > *
    > * this is id allocator without id -> pointer translation. Memory
    > * usage is much lower than full blown idr because each id only
    > * occupies a bit. ida uses a custom leaf node which contains
    > * IDA_BITMAP_BITS slots.
    > *
    > * 2007-04-25 written by Tejun Heo <htejun@gmail.com>
    > */
    >
    > static void free_bitmap(struct ida *ida, struct ida_bitmap *bitmap)
    > {
    > unsigned long flags;
    >
    > if (!ida->free_bitmap) {
    > spin_lock_irqsave(&ida->idr.lock, flags);
    > if (!ida->free_bitmap) {
    > ida->free_bitmap = bitmap;
    > bitmap = NULL;
    > }
    > spin_unlock_irqrestore(&ida->idr.lock, flags);
    > }
    >
    > kfree(bitmap);
    > }
    >
    > /**
    > * ida_pre_get - reserve resources for ida allocation
    > * @ida: ida handle
    > * @gfp_mask: memory allocation flag
    > *
    > * 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 ida_pre_get(struct ida *ida, gfp_t gfp_mask)
    > {
    > /* allocate idr_layers */
    > if (!idr_pre_get(&ida->idr, gfp_mask))
    > return 0;
    >
    > /* allocate free_bitmap */
    > if (!ida->free_bitmap) {
    > struct ida_bitmap *bitmap;
    >
    > bitmap = kmalloc(sizeof(struct ida_bitmap), gfp_mask);
    > if (!bitmap)
    > return 0;
    >
    > free_bitmap(ida, bitmap);
    > }
    >
    > return 1;
    > }
    > EXPORT_SYMBOL(ida_pre_get);
    >
    > /**
    > * ida_get_new_above - allocate new ID above or equal to a start id
    > * @ida: ida handle
    > * @staring_id: id to start search at
    > * @p_id: pointer to the allocated handle
    > *
    > * Allocate new ID above or equal to @ida. It should be called with
    > * any required locks.
    > *
    > * If memory is required, it will return -EAGAIN, you should unlock
    > * and go back to the ida_pre_get() call. If the ida is full, it will
    > * return -ENOSPC.
    > *
    > * @p_id returns a value in the range 0 ... 0x7fffffff.
    > */
    > int ida_get_new_above(struct ida *ida, int starting_id, int *p_id)
    > {
    > struct idr_layer *pa[MAX_LEVEL];
    > struct ida_bitmap *bitmap;
    > unsigned long flags;
    > int idr_id = starting_id / IDA_BITMAP_BITS;
    > int offset = starting_id % IDA_BITMAP_BITS;
    > int t, id;
    >
    > restart:
    > /* get vacant slot */
    > t = idr_get_empty_slot(&ida->idr, idr_id, pa);
    > if (t < 0)
    > return _idr_rc_to_errno(t);
    >
    > if (t * IDA_BITMAP_BITS >= MAX_ID_BIT)
    > return -ENOSPC;
    >
    > if (t != idr_id)
    > offset = 0;
    > idr_id = t;
    >
    > /* if bitmap isn't there, create a new one */
    > bitmap = (void *)pa[0]->ary[idr_id & IDR_MASK];

    OK, the caller presumably holds the update-side lock.

    > if (!bitmap) {
    > spin_lock_irqsave(&ida->idr.lock, flags);
    > bitmap = ida->free_bitmap;
    > ida->free_bitmap = NULL;
    > spin_unlock_irqrestore(&ida->idr.lock, flags);
    >
    > if (!bitmap)
    > return -EAGAIN;
    >
    > memset(bitmap, 0, sizeof(struct ida_bitmap));
    > rcu_assign_pointer(pa[0]->ary[idr_id & IDR_MASK],
    > (void *)bitmap);
    > pa[0]->count++;
    > }
    >
    > /* lookup for empty slot */
    > t = find_next_zero_bit(bitmap->bitmap, IDA_BITMAP_BITS, offset);
    > if (t == IDA_BITMAP_BITS) {
    > /* no empty slot after offset, continue to the next chunk */
    > idr_id++;
    > offset = 0;
    > goto restart;
    > }
    >
    > id = idr_id * IDA_BITMAP_BITS + t;
    > if (id >= MAX_ID_BIT)
    > return -ENOSPC;
    >
    > __set_bit(t, bitmap->bitmap);
    > if (++bitmap->nr_busy == IDA_BITMAP_BITS)
    > idr_mark_full(pa, idr_id);
    >
    > *p_id = id;
    >
    > /* Each leaf node can handle nearly a thousand slots and the
    > * whole idea of ida is to have small memory foot print.
    > * Throw away extra resources one by one after each successful
    > * allocation.
    > */
    > if (ida->idr.id_free_cnt || ida->free_bitmap) {
    > struct idr_layer *p = get_from_free_list(&ida->idr);
    > if (p)
    > kmem_cache_free(idr_layer_cache, p);
    > }
    >
    > return 0;
    > }
    > EXPORT_SYMBOL(ida_get_new_above);
    >
    > /**
    > * ida_get_new - allocate new ID
    > * @ida: idr handle
    > * @p_id: pointer to the allocated handle
    > *
    > * Allocate new ID. 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 ida_get_new(struct ida *ida, int *p_id)
    > {
    > return ida_get_new_above(ida, 0, p_id);
    > }
    > EXPORT_SYMBOL(ida_get_new);
    >
    > /**
    > * ida_remove - remove the given ID
    > * @ida: ida handle
    > * @id: ID to free
    > */
    > void ida_remove(struct ida *ida, int id)
    > {
    > struct idr_layer *p = ida->idr.top;
    > int shift = (ida->idr.layers - 1) * IDR_BITS;
    > int idr_id = id / IDA_BITMAP_BITS;
    > int offset = id % IDA_BITMAP_BITS;
    > int n;
    > struct ida_bitmap *bitmap;
    >
    > /* clear full bits while looking up the leaf idr_layer */
    > while ((shift > 0) && p) {
    > n = (idr_id >> shift) & IDR_MASK;
    > __clear_bit(n, &p->bitmap);
    > p = p->ary[n];

    OK, the caller presumably holds the update-side lock.

    > shift -= IDR_BITS;
    > }
    >
    > if (p == NULL)
    > goto err;
    >
    > n = idr_id & IDR_MASK;
    > __clear_bit(n, &p->bitmap);
    >
    > bitmap = (void *)p->ary[n];

    OK, the caller presumably holds the update-side lock.

    > if (!test_bit(offset, bitmap->bitmap))
    > goto err;
    >
    > /* update bitmap and remove it if empty */
    > __clear_bit(offset, bitmap->bitmap);
    > if (--bitmap->nr_busy == 0) {
    > __set_bit(n, &p->bitmap); /* to please idr_remove() */
    > idr_remove(&ida->idr, idr_id);
    > free_bitmap(ida, bitmap);
    > }
    >
    > return;
    >
    > err:
    > printk(KERN_WARNING
    > "ida_remove called for id=%d which is not allocated.\n", id);
    > }
    > EXPORT_SYMBOL(ida_remove);
    >
    > /**
    > * ida_destroy - release all cached layers within an ida tree
    > * ida: ida handle
    > */
    > void ida_destroy(struct ida *ida)
    > {
    > idr_destroy(&ida->idr);
    > kfree(ida->free_bitmap);
    > }
    > EXPORT_SYMBOL(ida_destroy);
    >
    > /**
    > * ida_init - initialize ida handle
    > * @ida: ida handle
    > *
    > * This function is use to set up the handle (@ida) that you will pass
    > * to the rest of the functions.
    > */
    > void ida_init(struct ida *ida)
    > {
    > memset(ida, 0, sizeof(struct ida));
    > idr_init(&ida->idr);
    >
    > }
    > EXPORT_SYMBOL(ida_init);


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