lru_cache: introduce lc_get_cumulative()

New helper to be able to consolidate more updates
into a single transaction.
Without this, we can only grab a single refcount
on an updated element while preparing a transaction.

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.

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

Fixed up by Jens to export lc_get_cumulative().

Signed-off-by: Jens Axboe <axboe@kernel.dk>

2 files changed, 47 insertions, 10 deletions

diff --git a/include/linux/lru_cache.h b/include/linux/lru_cache.h
index 4019013c6593..46262284de47 100644
--- a/include/linux/lru_cache.h
+++ b/include/linux/lru_cache.h
@@ -256,6 +256,7 @@ extern void lc_destroy(struct lru_cache *lc);
 extern void lc_set(struct lru_cache *lc, unsigned int enr, int index);
 extern void lc_del(struct lru_cache *lc, struct lc_element *element);
 
+extern struct lc_element *lc_get_cumulative(struct lru_cache *lc, unsigned int enr);
 extern struct lc_element *lc_try_get(struct lru_cache *lc, unsigned int enr);
 extern struct lc_element *lc_find(struct lru_cache *lc, unsigned int enr);
 extern struct lc_element *lc_get(struct lru_cache *lc, unsigned int enr);
diff --git a/lib/lru_cache.c b/lib/lru_cache.c
index 8335d39d2ccd..4a83ecd03650 100644
--- a/lib/lru_cache.c
+++ b/lib/lru_cache.c
@@ -365,7 +365,13 @@ static int lc_unused_element_available(struct lru_cache *lc)
         return 0;
 }
 
-static struct lc_element *__lc_get(struct lru_cache *lc, unsigned int enr, bool may_change)
+/* 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;
 
@@ -380,22 +386,31 @@ static struct lc_element *__lc_get(struct lru_cache *lc, unsigned int enr, bool
          * 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) {
+        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 (!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)
+        if (!(flags & LC_GET_MAY_CHANGE))
                 RETURN(NULL);
 
         /* To avoid races with lc_try_lock(), first, mark us dirty
@@ -477,7 +492,27 @@ static struct lc_element *__lc_get(struct lru_cache *lc, unsigned int enr, bool
  */
 struct lc_element *lc_get(struct lru_cache *lc, unsigned int enr)
 {
-        return __lc_get(lc, enr, 1);
+        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);
 }
 
 /**
@@ -648,3 +683,4 @@ 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);