path: root/lib/lru_cache.c

// 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
 * @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 */
	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\n",
		   lc->name, lc->used, lc->nr_elements,
		   lc->hits, lc->misses, lc->starving, lc->