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Subject[PATCH 12/18] rhashtable: add rhashtable_walk_prev()
rhashtable_walk_prev() 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_prev() 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@suse.com>
---
include/linux/rhashtable.h | 1 +
lib/rhashtable.c | 31 +++++++++++++++++++++++++++++++
2 files changed, 32 insertions(+)

diff --git a/include/linux/rhashtable.h b/include/linux/rhashtable.h
index c612d2446d90..1b70d690ab65 100644
--- a/include/linux/rhashtable.h
+++ b/include/linux/rhashtable.h
@@ -247,6 +247,7 @@ static inline void rhashtable_walk_start(struct rhashtable_iter *iter)
}

void *rhashtable_walk_next(struct rhashtable_iter *iter);
+void *rhashtable_walk_prev(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 5ab0f4825271..a185f5a8b34d 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -915,6 +915,37 @@ void *rhashtable_walk_next(struct rhashtable_iter *iter)
}
EXPORT_SYMBOL_GPL(rhashtable_walk_next);

+/**
+ * rhashtable_walk_prev - 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 and in particular it does not
+ * step backwards.
+ */
+void *rhashtable_walk_prev(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_prev);
+
/**
* rhashtable_walk_stop - Finish a hash table walk
* @iter: Hash table iterator

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