[PATCH - RFC] rhashtable: add rhashtable_walk_last_seen()

[NeilBrown]

rhashtable_walk_last_seen() returns the object returned by
the previous rhashtable_walk_next(), providing it is still in the
table (or was during this grace period).
This works even if rhashtable_walk_stop() and rhashtable_talk_start()
have been called since the last rhashtable_walk_next().

If there have been no calls to rhashtable_walk_next(), or if the
object is gone from the table, then NULL is returned.

This can usefully be used in a seq_file ->start() function.
If the pos is the same as was returned by the last ->next() call,
then rhashtable_walk_last_seen() can be used to re-establish the
current location in the table.  If it returns NULL, then
rhashtable_walk_next() should be used.

Signed-off-by: NeilBrown <neilb@xxxxxxxx>
---
 include/linux/rhashtable.h |  1 +
 lib/rhashtable.c           | 30 ++++++++++++++++++++++++++++++
 2 files changed, 31 insertions(+)

diff --git a/include/linux/rhashtable.h b/include/linux/rhashtable.h
index 657e37ae314c..d63b472e9d50 100644
--- a/include/linux/rhashtable.h
+++ b/include/linux/rhashtable.h
@@ -248,6 +248,7 @@ static inline void rhashtable_walk_start(struct rhashtable_iter *iter)
 
 void *rhashtable_walk_next(struct rhashtable_iter *iter);
 void *rhashtable_walk_peek(struct rhashtable_iter *iter);
+void *rhashtable_walk_last_seen(struct rhashtable_iter *iter);
 void rhashtable_walk_stop(struct rhashtable_iter *iter) __releases(RCU);
 
 void rhashtable_free_and_destroy(struct rhashtable *ht,
diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index e0060cb264e9..45f2554399a5 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -949,6 +949,36 @@ void *rhashtable_walk_peek(struct rhashtable_iter *iter)
 }
 EXPORT_SYMBOL_GPL(rhashtable_walk_peek);
 
+/**
+ * rhashtable_walk_last_seen - Return the previously returned object, if available
+ * @iter:	Hash table iterator
+ *
+ * If rhashtable_walk_next() has previously been called and the object
+ * it returned is still in the hash table, that object is returned again,
+ * otherwise %NULL is returned.
+ *
+ * If the recent rhashtable_walk_next() call was since the most recent
+ * rhashtable_walk_start() call then the returned object may not, strictly
+ * speaking, still be in the table.  It will be safe to dereference.
+ *
+ * Note that the iterator is not changed.
+ */
+void *rhashtable_walk_last_seen(struct rhashtable_iter *iter)
+{
+	struct rhashtable *ht = iter->ht;
+	struct rhash_head *p = iter->p;
+
+	if (!p)
+		return NULL;
+	if (!iter->p_is_unsafe || ht->rhlist)
+		return p;
+	rht_for_each_rcu(p, iter->walker.tbl, iter->slot)
+		if (p == iter->p)
+			return p;
+	return NULL;
+}
+EXPORT_SYMBOL_GPL(rhashtable_walk_last_seen);
+
 /**
  * rhashtable_walk_stop - Finish a hash table walk
  * @iter:	Hash table iterator
-- 
2.14.0.rc0.dirty

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