netdev - [PATCH 1/6] rhashtable: improve documentation for rhashtable_walk

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Date:   Tue, 27 Mar 2018 10:33:04 +1100
From:   NeilBrown <neilb@...e.com>
To:     Thomas Graf <tgraf@...g.ch>,
        Herbert Xu <herbert@...dor.apana.org.au>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 1/6] 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.  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 the one user will
be affected by the code change.

Signed-off-by: NeilBrown <neilb@...e.com>
---
 lib/rhashtable.c |   17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index 3825c30aaa36..24a57ca494cb 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)
 {
@@ -880,7 +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);
 }