[PATCH 1/6 v2] rhashtable: improve documentation for rhashtable_walk_peek()

[NeilBrown]

In this version I:
 - fixed brace-after-else coding style, thanks Sergei Shtylyov,
 - explained where the one user is, thanks David Miller
 - added CC for author of rhashtable_walk_peek (Tom Herbert) and
   of the one usage (Andreas Gruenbacher) - thanks Herbert Xu

NeilBrown

-----------------8<-------------------------
Subject: [PATCH] rhashtable: improve documentation for rhashtable_walk_peek()

The documentation for rhashtable_walk_peek() wrong.  It claims to
return the *next* entry, whereas it in fact returns the *previous*
entry.
However if no entries have yet been returned - or if the iterator
was reset due to a resize event, then rhashtable_walk_peek()
*does* return the next entry, but also advances the iterator.

I suspect that this interface should be discarded and the one user
should be changed to not require it.  The only current user is
gfs2_glock_iter_next in fs/gfs2/glock.c, which is part of the
implementation of a seq_file which reports all the content of the
hash table.
Possibly this patch should be seen as a first step in that
conversation.

This patch mostly corrects the documentation, but does make a
small code change so that the documentation can be correct without
listing too many special cases.  I don't think gfs2_glock_iter_next
will be affected by the code change.

Cc: Tom Herbert <tom@xxxxxxxxxxxxxx>
Cc: Andreas Gruenbacher <agruenba@xxxxxxxxxx>
Signed-off-by: NeilBrown <neilb@xxxxxxxx>
---
 lib/rhashtable.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index 3825c30aaa36..9367816820ea 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -853,13 +853,17 @@ void *rhashtable_walk_next(struct rhashtable_iter *iter)
 EXPORT_SYMBOL_GPL(rhashtable_walk_next);
 
 /**
- * rhashtable_walk_peek - Return the next object but don't advance the iterator
+ * rhashtable_walk_peek - Return the previously returned object without advancing the iterator
  * @iter:	Hash table iterator
  *
- * Returns the next object or NULL when the end of the table is reached.
+ * Returns the last object returned, or NULL if no object has yet been returned.
+ * If the previously returned object has since been removed, then some other arbitrary
+ * object maybe returned, or possibly NULL will be returned.  In that case, the
+ * iterator might be advanced.
  *
  * Returns -EAGAIN if resize event occurred.  Note that the iterator
- * will rewind back to the beginning and you may continue to use it.
+ * will rewind back to the beginning and rhashtable_walk_next() should be
+ * used to get the next object.
  */
 void *rhashtable_walk_peek(struct rhashtable_iter *iter)
 {
@@ -880,6 +884,12 @@ void *rhashtable_walk_peek(struct rhashtable_iter *iter)
 		 * the table hasn't changed.
 		 */
 		iter->skip--;
+	} else {
+		/* ->skip is only zero after rhashtable_walk_start()
+		 * or when the iterator is reset.  In this case there
+		 * is no previous object to return.
+		 */
+		return NULL;
 	}
 
 	return __rhashtable_walk_find_next(iter);
-- 
2.14.0.rc0.dirty

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