linux-kernel - [PATCH - RFC] rhashtable: implement rhashtable_walk_peek() using rhashtable_walk_last

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Message-ID: <87y3frmhyx.fsf@notabene.neil.brown.name>
Date:   Thu, 07 Jun 2018 12:46:30 +1000
From:   NeilBrown <neilb@...e.com>
To:     Tom Herbert <tom@...bertland.com>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        Thomas Graf <tgraf@...g.ch>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Tom Herbert <tom@...ntonium.net>
Subject: [PATCH - RFC] rhashtable: implement rhashtable_walk_peek() using
 rhashtable_walk_last_seen()


rhashtable_walk_last_seen() does most of the work that
rhashtable_walk_peek() needs done, so use it.
Also update the documentation for rhashtable_walk_peek() to clarify
the expected use case.

Signed-off-by: NeilBrown <neilb@...e.com>
---
 lib/rhashtable.c | 42 ++++++++++++++++++++----------------------
 1 file changed, 20 insertions(+), 22 deletions(-)

diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index 45f2554399a5..30bb9ead15f4 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -916,36 +916,34 @@ 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 next object to use in an interrupted walk
  * @iter:	Hash table iterator
  *
- * Returns the next object or NULL when the end of the table is reached.
+ * Returns the "current" object or NULL when the end of the table is reached.
+ * When an rhashtable_walk is interrupted with rhashtable_walk_stop(),
+ * it is often because an object was found that could not be processed
+ * immediately, possible because there is no more space to encode details
+ * of the object (e.g. when producing a seq_file from the table).
+ * When the walk is restarted, the same object needs to be processed again,
+ * if possible.  The object might have been removed from the table while
+ * the walk was paused, so it might not be available.  In that case, the
+ * normal "next" object should be treated as "current".
  *
- * Returns -EAGAIN if resize event occurred.  Note that the iterator
+ * To support this common case, rhashtable_walk_peek() returns the
+ * appropriate object to process after an interrupted walk, either the
+ * one that was most recently returned, or if that doesn't exist - the
+ * next one.
+ *
+ * Returns -EAGAIN if resize event occurred.  In that case the iterator
  * will rewind back to the beginning and you may continue to use it.
  */
 void *rhashtable_walk_peek(struct rhashtable_iter *iter)
 {
-	struct rhlist_head *list = iter->list;
-	struct rhashtable *ht = iter->ht;
-	struct rhash_head *p = iter->p;
+	void *ret = rhashtable_walk_last_seen(iter);
 
-	if (p)
-		return rht_obj(ht, ht->rhlist ? &list->rhead : p);
-
-	/* No object found in current iter, find next one in the table. */
-
-	if (iter->skip) {
-		/* A nonzero skip value points to the next entry in the table
-		 * beyond that last one that was found. Decrement skip so
-		 * we find the current value. __rhashtable_walk_find_next
-		 * will restore the original value of skip assuming that
-		 * the table hasn't changed.
-		 */
-		iter->skip--;
-	}
-
-	return __rhashtable_walk_find_next(iter);
+	if (!ret)
+		ret = rhashtable_walk_next(iter);
+	return ret;
 }
 EXPORT_SYMBOL_GPL(rhashtable_walk_peek);
 
-- 
2.14.0.rc0.dirty


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