/*
   lru_cache.c

   This file is part of DRBD by Philipp Reisner and Lars Ellenberg.

   Copyright (C) 2003-2008, LINBIT Information Technologies GmbH.
   Copyright (C) 2003-2008, Philipp Reisner <philipp.reisner@linbit.com>.
   Copyright (C) 2003-2008, Lars Ellenberg <lars.ellenberg@linbit.com>.

   drbd is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2, or (at your option)
   any later version.

   drbd is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with drbd; see the file COPYING.  If not, write to
   the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.

 */

#include <linux/module.h>
#include <linux/bitops.h>
#include <linux/slab.h>
#include <linux/string.h> /* for memset */
#include <linux/seq_file.h> /* for seq_printf */
#include <linux/lru_cache.h>

MODULE_AUTHOR("Philipp Reisner <phil@linbit.com>, "
              "Lars Ellenberg <lars@linbit.com>");
MODULE_DESCRIPTION("lru_cache - Track sets of hot objects");
MODULE_LICENSE("GPL");

/* this is developers aid only.
 * it catches concurrent access (lack of locking on the users part) */
#define PARANOIA_ENTRY() do {           \
        BUG_ON(!lc);                    \
        BUG_ON(!lc->nr_elements);       \
        BUG_ON(test_and_set_bit(__LC_PARANOIA, &lc->flags)); \
} while (0)

#define RETURN(x...)     do { \
        clear_bit_unlock(__LC_PARANOIA, &lc->flags); \
        return x ; } while (0)

/* BUG() if e is not one of the elements tracked by lc */
#define PARANOIA_LC_ELEMENT(lc, e) do { \
        struct lru_cache *lc_ = (lc);   \
        struct lc_element *e_ = (e);    \
        unsigned i = e_->lc_index;      \
        BUG_ON(i >= lc_->nr_elements);  \
        BUG_ON(lc_->lc_element[i] != e_); } while (0)


/* We need to atomically
 *  - try to grab the lock (set LC_LOCKED)
 *  - only if there is no pending transaction
 *    (neither LC_DIRTY nor LC_STARVING is set)
 * Because of PARANOIA_ENTRY() above abusing lc->flags as well,
 * it is not sufficient to just say
 *      return 0 == cmpxchg(&lc->flags, 0, LC_LOCKED);
 */
int lc_try_lock(struct lru_cache *lc)
{
        unsigned long val;
        do {
                val = cmpxchg(&lc->flags, 0, LC_LOCKED);
        } while (unlikely (val == LC_PARANOIA));
        /* Spin until no-one is inside a PARANOIA_ENTRY()/RETURN() section. */
        return 0 == val;
#if 0
        /* Alternative approach, spin in case someone enters or leaves a
         * PARANOIA_ENTRY()/RETURN() section. */
        unsigned long old, new, val;
        do {
                old = lc->flags & LC_PARANOIA;
                new = old | LC_LOCKED;
                val = cmpxchg(&lc->flags, old, new);
        } while (unlikely (val == (old ^ LC_PARANOIA)));
        return old == val;
#endif
}

/**
 * lc_create - prepares to track objects in an active set
 * @name: descriptive name only used in lc_seq_printf_stats and lc_seq_dump_details
 * @max_pending_changes: maximum changes to accumulate until a transaction is required
 * @e_count: number of elements allowed to be active simultaneously
 * @e_size: size of the tracked objects
 * @e_off: offset to the &struct lc_element member in a tracked object
 *
 * Returns a pointer to a newly initialized struct lru_cache on success,
 * or NULL on (allocation) failure.
 */
struct lru_cache *lc_create(const char *name, struct kmem_cache *cache,
                unsigned max_pending_changes,
                unsigned e_count, size_t e_size, size_t e_off)
{
        struct hlist_head *slot = NULL;
        struct lc_element **element = NULL;
        struct lru_cache *lc;
        struct lc_element *e;
        unsigned cache_obj_size = kmem_cache_size(cache);
        unsigned i;

        WARN_ON(cache_obj_size < e_size);
        if (cache_obj_size < e_size)
                return NULL;

        /* e_count too big; would probably fail the allocation below anyways.
         * for typical use cases, e_count should be few thousand at most. */
        if (e_count > LC_MAX_ACTIVE)
                return NULL;

        slot = kcalloc(e_count, sizeof(struct hlist_head), GFP_KERNEL);
        if (!slot)
                goto out_fail;
        element = kzalloc(e_count * sizeof(struct lc_element *), GFP_KERNEL);
        if (!element)
                goto out_fail;

        lc = kzalloc(sizeof(*lc), GFP_KERNEL);
        if (!lc)
                goto out_fail;

        INIT_LIST_HEAD(&lc->in_use);
        INIT_LIST_HEAD(&lc->lru);
        INIT_LIST_HEAD(&lc->free);
        INIT_LIST_HEAD(&lc->to_be_changed);

        lc->name = name;
        lc->element_size = e_size;
        lc->element_off = e_off;
        lc->nr_elements = e_count;
        lc->max_pending_changes = max_pending_changes;
        lc->lc_cache = cache;
        lc->lc_element = element;
        lc->lc_slot = slot;

        /* preallocate all objects */
        for (i = 0; i < e_count; i++) {
                void *p = kmem_cache_alloc(cache, GFP_KERNEL);
                if (!p)
                        break;
                memset(p, 0, lc->element_size);
                e = p + e_off;
                e->lc_index = i;
                e->lc_number = LC_FREE;
                e->lc_new_number = LC_FREE;
                list_add(&e->list, &lc->free);
                element[i] = e;
        }
        if (i == e_count)
                return lc;

        /* else: could not allocate all elements, give up */
        for (i--; i; i--) {
                void *p = element[i];
                kmem_cache_free(cache, p - e_off);
        }
        kfree(lc);
out_fail:
        kfree(element);
        kfree(slot);
        return NULL;
}

void lc_free_by_index(struct lru_cache *lc, unsigned i)
{
        void *p = lc->lc_element[i];
        WARN_ON(!p);
        if (p) {
                p -= lc->element_off;
                kmem_cache_free(lc->lc_cache, p);
        }
}

/**
 * lc_destroy - frees memory allocated by lc_create()
 * @lc: the lru cache to destroy
 */
void lc_destroy(struct lru_cache *lc)
{
        unsigned i;
        if (!lc)
                return;
        for (i = 0; i < lc->nr_elements; i++)
                lc_free_by_index(lc, i);
        kfree(lc->lc_element);
        kfree(lc->lc_slot);
        kfree(lc);
}

/**
 * lc_reset - does a full reset for @lc and the hash table slots.
 * @lc: the lru cache to operate on
 *
 * It is roughly the equivalent of re-allocating a fresh lru_cache object,
 * basically a short cut to lc_destroy(lc); lc = lc_create(...);
 */
void lc_reset(struct lru_cache *lc)
{
        unsigned i;

        INIT_LIST_HEAD(&lc->in_use);
        INIT_LIST_HEAD(&lc->lru);
        INIT_LIST_HEAD(&lc->free);
        INIT_LIST_HEAD(&lc->to_be_changed);
        lc->used = 0;
        lc->hits = 0;
        lc->misses = 0;
        lc->starving = 0;
        lc->locked = 0;
        lc->changed = 0;
        lc->pending_changes = 0;
        lc->flags = 0;
        memset(lc->lc_slot, 0, sizeof(struct hlist_head) * lc->nr_elements);

        for (i = 0; i < lc->nr_elements; i++) {
                struct lc_element *e = lc->lc_element[i];
                void *p = e;
                p -= lc->element_off;
                memset(p, 0, lc->element_size);
                /* re-init it */
                e->lc_index = i;
                e->lc_number = LC_FREE;
                e->lc_new_number = LC_FREE;
                list_add(&e->list, &lc->free);
        }
}

/**
 * lc_seq_printf_stats - print stats about @lc into @seq
 * @seq: the seq_file to print into
 * @lc: the lru cache to print statistics of
 */
size_t lc_seq_printf_stats(struct seq_file *seq, struct lru_cache *lc)
{
        /* NOTE:
         * total calls to lc_get are
         * (starving + hits + misses)
         * misses include "locked" count (update from an other thread in
         * progress) and "changed", when this in fact lead to an successful
         * update of the cache.
         */
        return seq_printf(seq, "\t%s: used:%u/%u "
                "hits:%lu misses:%lu starving:%lu locked:%lu changed:%lu\n",
                lc->name, lc->used, lc->nr_elements,
                lc->hits, lc->misses, lc->starving, lc->locked, lc->changed);
}

static struct hlist_head *lc_hash_slot(struct lru_cache *lc, unsigned int enr)
{
        return  lc->lc_slot + (enr % lc->nr_elements);
}


static struct lc_element *__lc_find(struct lru_cache *lc, unsigned int enr,
                bool include_changing)
{
        struct hlist_node *n;
        struct lc_element *e;

        BUG_ON(!lc);
        BUG_ON(!lc->nr_elements);
        hlist_for_each_entry(e, n, lc_hash_slot(lc, enr), colision) {
                /* "about to be changed" elements, pending transaction commit,
                 * are hashed by their "new number". "Normal" elements have
                 * lc_number == lc_new_number. */
                if (e->lc_new_number != enr)
                        continue;
                if (e->lc_new_number == e->lc_number || include_changing)
                        return e;
                break;
        }
        return NULL;
}

/**
 * lc_find - find element by label, if present in the hash table
 * @lc: The lru_cache object
 * @enr: element number
 *
 * Returns the pointer to an element, if the element with the requested
 * "label" or element number is present in the hash table,
 * or NULL if not found. Does not change the refcnt.
 * Ignores elements that are "about to be used", i.e. not yet in the active
 * set, but still pending transaction commit.
 */
struct lc_element *lc_find(struct lru_cache *lc, unsigned int enr)
{
        return __lc_find(lc, enr, 0);
}

/**
 * lc_is_used - find element by label
 * @lc: The lru_cache object
 * @enr: element number
 *
 * Returns true, if the element with the requested "label" or element number is
 * present in the hash table, and is used (refcnt > 0).
 * Also finds elements that are not _currently_ used but only "about to be
 * used", i.e. on the "to_be_changed" list, pending transaction commit.
 */
bool lc_is_used(struct lru_cache *lc, unsigned int enr)
{
        struct lc_element *e = __lc_find(lc, enr, 1);
        return e && e->refcnt;
}

/**
 * lc_del - removes an element from the cache
 * @lc: The lru_cache object
 * @e: The element to remove
 *
 * @e must be unused (refcnt == 0). Moves @e from "lru" to "free" list,
 * sets @e->enr to %LC_FREE.
 */
void lc_del(struct lru_cache *lc, struct lc_element *e)
{
        PARANOIA_ENTRY();
        PARANOIA_LC_ELEMENT(lc, e);
        BUG_ON(e->refcnt);

        e->lc_number = e->lc_new_number = LC_FREE;
        hlist_del_init(&e->colision);
        list_move(&e->list, &lc->free);
        RETURN();
}

static struct lc_element *lc_prepare_for_change(struct lru_cache *lc, unsigned new_number)
{
        struct list_head *n;
        struct lc_element *e;

        if (!list_empty(&lc->free))
                n = lc->free.next;
        else if (!list_empty(&lc->lru))
                n = lc->lru.prev;
        else
                return NULL;

        e = list_entry(n, struct lc_element, list);
        PARANOIA_LC_ELEMENT(lc, e);

        e->lc_new_number = new_number;
        if (!hlist_unhashed(&e->colision))
                __hlist_del(&e->colision);
        hlist_add_head(&e->colision, lc_hash_slot(lc, new_number));
        list_move(&e->list, &lc->to_be_changed);

        return e;
}

static int lc_unused_element_available(struct lru_cache *lc)
{
        if (!list_empty(&lc->free))
                return 1; /* something on the free list */
        if (!list_empty(&lc->lru))
                return 1;  /* something to evict */

        return 0;
}

static struct lc_element *__lc_get(struct lru_cache *lc, unsigned int enr, bool may_change)
{
        struct lc_element *e;

        PARANOIA_ENTRY();
        if (lc->flags & LC_STARVING) {
                ++lc->starving;
                RETURN(NULL);
        }

        e = __lc_find(lc, enr, 1);
        /* if lc_new_number != lc_number,
         * this enr is currently being pulled in already,
         * and will be available once the pending transaction
         * has been committed. */
        if (e && e->lc_new_number == e->lc_number) {
                ++lc->hits;
                if (e->refcnt++ == 0)
                        lc->used++;
                list_move(&e->list, &lc->in_use); /* Not evictable... */
                RETURN(e);
        }

        ++lc->misses;
        if (!may_change)
                RETURN(NULL);

        /* It has been found above, but on the "to_be_changed" list, not yet
         * committed.  Don't pull it in twice, wait for the transaction, then
         * try again */
        if (e)
                RETURN(NULL);

        /* To avoid races with lc_try_lock(), first, mark us dirty
         * (using test_and_set_bit, as it implies memory barriers), ... */
        test_and_set_bit(__LC_DIRTY, &lc->flags);

        /* ... only then check if it is locked anyways. If lc_unlock clears
         * the dirty bit again, that's not a problem, we will come here again.
         */
        if (test_bit(__LC_LOCKED, &lc->flags)) {
                ++lc->locked;
                RETURN(NULL);
        }

        /* In case there is nothing available and we can not kick out
         * the LRU element, we have to wait ...
         */
        if (!lc_unused_element_available(lc)) {
                __set_bit(__LC_STARVING, &lc->flags);
                RETURN(NULL);
        }

        /* It was not present in the active set.  We are going to recycle an
         * unused (or even "free") element, but we won't accumulate more than
         * max_pending_changes changes.  */
        if (lc->pending_changes >= lc->max_pending_changes)
                RETURN(NULL);

        e = lc_prepare_for_change(lc, enr);
        BUG_ON(!e);

        clear_bit(__LC_STARVING, &lc->flags);
        BUG_ON(++e->refcnt != 1);
        lc->used++;
        lc->pending_changes++;

        RETURN(e);
}

/**
 * lc_get - get element by label, maybe change the active set
 * @lc: the lru cache to operate on
 * @enr: the label to look up
 *
 * Finds an element in the cache, increases its usage count,
 * "touches" and returns it.
 *
 * In case the requested number is not present, it needs to be added to the
 * cache. Therefore it is possible that an other element becomes evicted from
 * the cache. In either case, the user is notified so he is able to e.g. keep
 * a persistent log of the cache changes, and therefore the objects in use.
 *
 * Return values:
 *  NULL
 *     The cache was marked %LC_STARVING,
 *     or the requested label was not in the active set
 *     and a changing transaction is still pending (@lc was marked %LC_DIRTY).
 *     Or no unused or free element could be recycled (@lc will be marked as
 *     %LC_STARVING, blocking further lc_get() operations).
 *
 *  pointer to the element with the REQUESTED element number.
 *     In this case, it can be used right away
 *
 *  pointer to an UNUSED element with some different element number,
 *          where that different number may also be %LC_FREE.
 *
 *          In this case, the cache is marked %LC_DIRTY,
 *          so lc_try_lock() will no longer succeed.
 *          The returned element pointer is moved to the "to_be_changed" list,
 *          and registered with the new element number on the hash collision chains,
 *          so it is possible to pick it up from lc_is_used().
 *          Up to "max_pending_changes" (see lc_create()) can be accumulated.
 *          The user now should do whatever housekeeping is necessary,
 *          typically serialize on lc_try_lock_for_transaction(), then call
 *          lc_committed(lc) and lc_unlock(), to finish the change.
 *
 * NOTE: The user needs to check the lc_number on EACH use, so he recognizes
 *       any cache set change.
 */
struct lc_element *lc_get(struct lru_cache *lc, unsigned int enr)
{
        return __lc_get(lc, enr, 1);
}

/**
 * lc_try_get - get element by label, if present; do not change the active set
 * @lc: the lru cache to operate on
 * @enr: the label to look up
 *
 * Finds an element in the cache, increases its usage count,
 * "touches" and returns it.
 *
 * Return values:
 *  NULL
 *     The cache was marked %LC_STARVING,
 *     or the requested label was not in the active set
 *
 *  pointer to the element with the REQUESTED element number.
 *     In this case, it can be used right away
 */
struct lc_element *lc_try_get(struct lru_cache *lc, unsigned int enr)
{
        return __lc_get(lc, enr, 0);
}

/**
 * lc_committed - tell @lc that pending changes have been recorded
 * @lc: the lru cache to operate on
 *
 * User is expected to serialize on explicit lc_try_lock_for_transaction()
 * before the transaction is started, and later needs to lc_unlock() explicitly
 * as well.
 */
void lc_committed(struct lru_cache *lc)
{
        struct lc_element *e, *tmp;

        PARANOIA_ENTRY();
        list_for_each_entry_safe(e, tmp, &lc->to_be_changed, list) {
                /* count number of changes, not number of transactions */
                ++lc->changed;
                e->lc_number = e->lc_new_number;
                list_move(&e->list, &lc->in_use);
        }
        lc->pending_changes = 0;
        RETURN();
}


/**
 * lc_put - give up refcnt of @e
 * @lc: the lru cache to operate on
 * @e: the element to put
 *
 * If refcnt reaches zero, the element is moved to the lru list,
 * and a %LC_STARVING (if set) is cleared.
 * Returns the new (post-decrement) refcnt.
 */
unsigned int lc_put(struct lru_cache *lc, struct lc_element *e)
{
        PARANOIA_ENTRY();
        PARANOIA_LC_ELEMENT(lc, e);
        BUG_ON(e->refcnt == 0);
        BUG_ON(e->lc_number != e->lc_new_number);
        if (--e->refcnt == 0) {
                /* move it to the front of LRU. */
                list_move(&e->list, &lc->lru);
                lc->used--;
                clear_bit_unlock(__LC_STARVING, &lc->flags);
        }
        RETURN(e->refcnt);
}

/**
 * lc_element_by_index
 * @lc: the lru cache to operate on
 * @i: the index of the element to return
 */
struct lc_element *lc_element_by_index(struct lru_cache *lc, unsigned i)
{
        BUG_ON(i >= lc->nr_elements);
        BUG_ON(lc->lc_element[i] == NULL);
        BUG_ON(lc->lc_element[i]->lc_index != i);
        return lc->lc_element[i];
}

/**
 * lc_index_of
 * @lc: the lru cache to operate on
 * @e: the element to query for its index position in lc->element
 */
unsigned int lc_index_of(struct lru_cache *lc, struct lc_element *e)
{
        PARANOIA_LC_ELEMENT(lc, e);
        return e->lc_index;
}

/**
 * lc_set - associate index with label
 * @lc: the lru cache to operate on
 * @enr: the label to set
 * @index: the element index to associate label with.
 *
 * Used to initialize the active set to some previously recorded state.
 */
void lc_set(struct lru_cache *lc, unsigned int enr, int index)
{
        struct lc_element *e;
        struct list_head *lh;

        if (index < 0 || index >= lc->nr_elements)
                return;

        e = lc_element_by_index(lc, index);
        BUG_ON(e->lc_number != e->lc_new_number);
        BUG_ON(e->refcnt != 0);

        e->lc_number = e->lc_new_number = enr;
        hlist_del_init(&e->colision);
        if (enr == LC_FREE)
                lh = &lc->free;
        else {
                hlist_add_head(&e->colision, lc_hash_slot(lc, enr));
                lh = &lc->lru;
        }
        list_move(&e->list, lh);
}

/**
 * lc_dump - Dump a complete LRU cache to seq in textual form.
 * @lc: the lru cache to operate on
 * @seq: the &struct seq_file pointer to seq_printf into
 * @utext: user supplied "heading" or other info
 * @detail: function pointer the user may provide to dump further details
 * of the object the lc_element is embedded in.
 */
void lc_seq_dump_details(struct seq_file *seq, struct lru_cache *lc, char *utext,
             void (*detail) (struct seq_file *, struct lc_element *))
{
        unsigned int nr_elements = lc->nr_elements;
        struct lc_element *e;
        int i;

        seq_printf(seq, "\tnn: lc_number refcnt %s\n ", utext);
        for (i = 0; i < nr_elements; i++) {
                e = lc_element_by_index(lc, i);
                if (e->lc_number == LC_FREE) {
                        seq_printf(seq, "\t%2d: FREE\n", i);
                } else {
                        seq_printf(seq, "\t%2d: %4u %4u    ", i,
                                   e->lc_number, e->refcnt);
                        detail(seq, e);
                }
        }
}

EXPORT_SYMBOL(lc_create);
EXPORT_SYMBOL(lc_reset);
EXPORT_SYMBOL(lc_destroy);
EXPORT_SYMBOL(lc_set);
EXPORT_SYMBOL(lc_del);
EXPORT_SYMBOL(lc_try_get);
EXPORT_SYMBOL(lc_find);
EXPORT_SYMBOL(lc_get);
EXPORT_SYMBOL(lc_put);
EXPORT_SYMBOL(lc_committed);
EXPORT_SYMBOL(lc_element_by_index);
EXPORT_SYMBOL(lc_index_of);
EXPORT_SYMBOL(lc_seq_printf_stats);
EXPORT_SYMBOL(lc_seq_dump_details);
EXPORT_SYMBOL(lc_try_lock);
EXPORT_SYMBOL(lc_is_used);