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 // SPDX-License-Identifier: GPL-2.0-or-later
 /*
    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>.
 
 
  */
 
 #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
  * @cache: cache root pointer
  * @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 = kcalloc(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 */
     while (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\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 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_seq_dump_details - 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 '\n' 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\n ", 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, '\n');
     }
 }
 
 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);

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