lru_cache: allow multiple changes per transaction

Allow multiple changes to the active set of elements in lru_cache.
The only current user of lru_cache, drbd, is driving this generalisation.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

1 files changed, 167 insertions, 76 deletions

diff --git a/lib/lru_cache.c b/lib/lru_cache.c
index 17621684758a..d71d89498943 100644
--- a/lib/lru_cache.c
+++ b/lib/lru_cache.c
@@ -55,9 +55,40 @@ MODULE_LICENSE("GPL");
         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
@@ -66,6 +97,7 @@ MODULE_LICENSE("GPL");
  * 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;
@@ -98,12 +130,13 @@ struct lru_cache *lc_create(const char *name, struct kmem_cache *cache,
         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->new_number = LC_FREE;
+        lc->max_pending_changes = max_pending_changes;
         lc->lc_cache = cache;
         lc->lc_element = element;
         lc->lc_slot = slot;
@@ -117,6 +150,7 @@ struct lru_cache *lc_create(const char *name, struct kmem_cache *cache,
                 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;
         }
@@ -175,15 +209,15 @@ void lc_reset(struct lru_cache *lc)
         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->dirty = 0;
+        lc->locked = 0;
         lc->changed = 0;
+        lc->pending_changes = 0;
         lc->flags = 0;
-        lc->changing_element = NULL;
-        lc->new_number = LC_FREE;
         memset(lc->lc_slot, 0, sizeof(struct hlist_head) * lc->nr_elements);
 
         for (i = 0; i < lc->nr_elements; i++) {
@@ -194,6 +228,7 @@ void lc_reset(struct lru_cache *lc)
                 /* re-init it */
                 e->lc_index = i;
                 e->lc_number = LC_FREE;
+                e->lc_new_number = LC_FREE;
                 list_add(&e->list, &lc->free);
         }
 }
@@ -208,14 +243,14 @@ 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 "dirty" count (update from an other thread in
+         * 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 dirty:%lu changed:%lu\n",
+                "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->dirty, lc->changed);
+                lc->hits, lc->misses, lc->starving, lc->locked, lc->changed);
 }
 
 static struct hlist_head *lc_hash_slot(struct lru_cache *lc, unsigned int enr)
@@ -224,16 +259,8 @@ static struct hlist_head *lc_hash_slot(struct lru_cache *lc, unsigned int enr)
 }
 
 
-/**
- * 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.
- */
-struct lc_element *lc_find(struct lru_cache *lc, unsigned int enr)
+static struct lc_element *__lc_find(struct lru_cache *lc, unsigned int enr,
+                bool include_changing)
 {
         struct hlist_node *n;
         struct lc_element *e;
@@ -241,29 +268,48 @@ struct lc_element *lc_find(struct lru_cache *lc, unsigned int enr)
         BUG_ON(!lc);
         BUG_ON(!lc->nr_elements);
         hlist_for_each_entry(e, n, lc_hash_slot(lc, enr), colision) {
-                if (e->lc_number == enr)
+                /* "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;
 }
 
-/* returned element will be "recycled" immediately */
-static struct lc_element *lc_evict(struct lru_cache *lc)
+/**
+ * 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)
 {
-        struct list_head  *n;
-        struct lc_element *e;
-
-        if (list_empty(&lc->lru))
-                return NULL;
-
-        n = lc->lru.prev;
-        e = list_entry(n, struct lc_element, list);
-
-        PARANOIA_LC_ELEMENT(lc, e);
+        return __lc_find(lc, enr, 0);
+}
 
-        list_del(&e->list);
-        hlist_del(&e->colision);
-        return e;
+/**
+ * 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;
 }
 
 /**
@@ -280,22 +326,34 @@ void lc_del(struct lru_cache *lc, struct lc_element *e)
         PARANOIA_LC_ELEMENT(lc, e);
         BUG_ON(e->refcnt);
 
-        e->lc_number = LC_FREE;
+        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_get_unused_element(struct lru_cache *lc)
+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);
 
-        if (list_empty(&lc->free))
-                return lc_evict(lc);
+        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);
 
-        n = lc->free.next;
-        list_del(n);
-        return list_entry(n, struct lc_element, list);
+        return e;
 }
 
 static int lc_unused_element_available(struct lru_cache *lc)
@@ -318,8 +376,12 @@ static struct lc_element *__lc_get(struct lru_cache *lc, unsigned int enr, bool
                 RETURN(NULL);
         }
 
-        e = lc_find(lc, enr);
-        if (e) {
+        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++;
@@ -331,6 +393,24 @@ static struct lc_element *__lc_get(struct lru_cache *lc, unsigned int enr, bool
         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 ...
          */
@@ -339,24 +419,19 @@ static struct lc_element *__lc_get(struct lru_cache *lc, unsigned int enr, bool
                 RETURN(NULL);
         }
 
-        /* it was not present in the active set.
-         * we are going to recycle an unused (or even "free") element.
-         * user may need to commit a transaction to record that change.
-         * we serialize on flags & LC_DIRTY */
-        if (test_and_set_bit(__LC_DIRTY, &lc->flags)) {
-                ++lc->dirty;
+        /* 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_get_unused_element(lc);
+        e = lc_prepare_for_change(lc, enr);
         BUG_ON(!e);
 
         clear_bit(__LC_STARVING, &lc->flags);
         BUG_ON(++e->refcnt != 1);
         lc->used++;
-
-        lc->changing_element = e;
-        lc->new_number = enr;
+        lc->pending_changes++;
 
         RETURN(e);
 }
@@ -388,12 +463,15 @@ static struct lc_element *__lc_get(struct lru_cache *lc, unsigned int enr, bool
  *  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 (blocking further changes),
- *          and the returned element pointer is removed from the lru list and
- *          hash collision chains.  The user now should do whatever housekeeping
- *          is necessary.
- *          Then he must call lc_changed(lc,element_pointer), to finish
- *          the change.
+ *          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.
@@ -425,22 +503,25 @@ struct lc_element *lc_try_get(struct lru_cache *lc, unsigned int enr)
 }
 
 /**
- * lc_changed - tell @lc that the change has been recorded
+ * lc_committed - tell @lc that pending changes have been recorded
  * @lc: the lru cache to operate on
- * @e: the element pending label change
+ *
+ * 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_changed(struct lru_cache *lc, struct lc_element *e)
+void lc_committed(struct lru_cache *lc)
 {
+        struct lc_element *e, *tmp;
+
         PARANOIA_ENTRY();
-        BUG_ON(e != lc->changing_element);
-        PARANOIA_LC_ELEMENT(lc, e);
-        ++lc->changed;
-        e->lc_number = lc->new_number;
-        list_add(&e->list, &lc->in_use);
-        hlist_add_head(&e->colision, lc_hash_slot(lc, lc->new_number));
-        lc->changing_element = NULL;
-        lc->new_number = LC_FREE;
-        clear_bit_unlock(__LC_DIRTY, &lc->flags);
+        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();
 }
 
@@ -459,7 +540,7 @@ 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->changing_element);
+        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);
@@ -504,16 +585,24 @@ unsigned int lc_index_of(struct lru_cache *lc, struct lc_element *e)
 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);
-        e->lc_number = enr;
+        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);
-        hlist_add_head(&e->colision, lc_hash_slot(lc, enr));
-        list_move(&e->list, e->refcnt ? &lc->in_use : &lc->lru);
+        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);
 }
 
 /**
@@ -553,8 +642,10 @@ EXPORT_SYMBOL(lc_try_get);
 EXPORT_SYMBOL(lc_find);
 EXPORT_SYMBOL(lc_get);
 EXPORT_SYMBOL(lc_put);
-EXPORT_SYMBOL(lc_changed);
+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);