/*
     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;
  }

  static 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
   */

  void 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.
  	 */

  	seq_printf(seq, "\t%s: used:%u/%u hits:%lu misses:%lu starving:%lu locked:%lu changed:%lu
  ",
  		   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 lc_element *e;
  
  	BUG_ON(!lc);
  	BUG_ON(!lc->nr_elements);

  	hlist_for_each_entry(e, 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;
  }

  /* used as internal flags to __lc_get */
  enum {
  	LC_GET_MAY_CHANGE = 1,
  	LC_GET_MAY_USE_UNCOMMITTED = 2,
  };
  
  static struct lc_element *__lc_get(struct lru_cache *lc, unsigned int enr, unsigned int flags)

  {
  	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) {
  		if (e->lc_new_number != e->lc_number) {
  			/* 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 (!(flags & LC_GET_MAY_USE_UNCOMMITTED))
  				RETURN(NULL);
  			/* ... unless the caller is aware of the implications,
  			 * probably preparing a cumulative transaction. */
  			++e->refcnt;
  			++lc->hits;
  			RETURN(e);
  		}
  		/* else: lc_new_number == lc_number; a real hit. */

  		++lc->hits;
  		if (e->refcnt++ == 0)
  			lc->used++;
  		list_move(&e->list, &lc->in_use); /* Not evictable... */
  		RETURN(e);
  	}

  	/* e == NULL */

  
  	++lc->misses;

  	if (!(flags & LC_GET_MAY_CHANGE))

  		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, LC_GET_MAY_CHANGE);
  }
  
  /**
   * lc_get_cumulative - like lc_get; also finds to-be-changed elements
   * @lc: the lru cache to operate on
   * @enr: the label to look up
   *
   * Unlike lc_get this also returns the element for @enr, if it is belonging to
   * a pending transaction, so the return values are like for lc_get(),
   * plus:
   *
   * pointer to an element already on the "to_be_changed" list.
   * 	In this case, the cache was already marked %LC_DIRTY.
   *
   * Caller needs to make sure that the pending transaction is completed,
   * before proceeding to actually use this element.
   */
  struct lc_element *lc_get_cumulative(struct lru_cache *lc, unsigned int enr)
  {
  	return __lc_get(lc, enr, LC_GET_MAY_CHANGE|LC_GET_MAY_USE_UNCOMMITTED);

  }

  /**
   * 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 additional "heading" or other info

   * @detail: function pointer the user may provide to dump further details

   * of the object the lc_element is embedded in. May be NULL.
   * Note: a leading space ' ' and trailing newline '
  ' is implied.

   */
  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 (new nr) refcnt %s
   ", utext);

  	for (i = 0; i < nr_elements; i++) {
  		e = lc_element_by_index(lc, i);

  		if (e->lc_number != e->lc_new_number)
  			seq_printf(seq, "\t%5d: %6d %8d %6d ",
  				i, e->lc_number, e->lc_new_number, e->refcnt);
  		else
  			seq_printf(seq, "\t%5d: %6d %-8s %6d ",
  				i, e->lc_number, "-\"-", e->refcnt);
  		if (detail)

  			detail(seq, e);

  		seq_putc(seq, '
  ');

  	}
  }
  
  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);

  EXPORT_SYMBOL(lc_get_cumulative);