[PATCH RFC v2] rhashtable: implement rhashtable_walk_peek() using rhashtable_walk_last_seen()

From: NeilBrown <neilb@suse.com>
To: Tom Herbert <tom@herbertland.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>,
	Thomas Graf <tgraf@suug.ch>,
	Linux Kernel Network Developers <netdev@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Tom Herbert <tom@quantonium.net>
Subject: [PATCH RFC v2] rhashtable: implement rhashtable_walk_peek() using rhashtable_walk_last_seen()
Date: Tue, 12 Jun 2018 12:48:31 +1000	[thread overview]
Message-ID: <871sdck9ds.fsf@notabene.neil.brown.name> (raw)
In-Reply-To: <CALx6S35GgUOd0dPgv7P96wNNTv5pN7fij0pcAoccqcSWZhvY7Q@mail.gmail.com>

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rhashtable_walk_last_seen() does most of the work that
rhashtable_walk_peek() needs done, so use it and put
it in a "static inline".
Also update the documentation for rhashtable_walk_peek() to clarify
the expected use case.

Signed-off-by: NeilBrown <neilb@suse.com>
---

v2 as static-inline - suggested by Tom.

Thanks,
NeilBrown

 include/linux/rhashtable.h | 29 ++++++++++++++++++++++++++++-
 lib/rhashtable.c           | 34 ----------------------------------
 2 files changed, 28 insertions(+), 35 deletions(-)

diff --git a/include/linux/rhashtable.h b/include/linux/rhashtable.h
index d63b472e9d50..96ebc2690027 100644
--- a/include/linux/rhashtable.h
+++ b/include/linux/rhashtable.h
@@ -247,10 +247,37 @@ 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);
 
+/**
+ * rhashtable_walk_peek - Return the next object to use in an interrupted walk
+ * @iter:	Hash table iterator
+ *
+ * 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".
+ *
+ * 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.
+ */
+static inline void *rhashtable_walk_peek(struct rhashtable_iter *iter)
+{
+	return rhashtable_walk_last_seen(iter) ?:
+		rhashtable_walk_next(iter);
+}
+
 void rhashtable_free_and_destroy(struct rhashtable *ht,
 				 void (*free_fn)(void *ptr, void *arg),
 				 void *arg);
diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index 45f2554399a5..354275037df3 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -915,40 +915,6 @@ 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
- * @iter:	Hash table iterator
- *
- * Returns the next object or NULL when the end of the table is reached.
- *
- * Returns -EAGAIN if resize event occurred.  Note that 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;
-
-	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);
-}
-EXPORT_SYMBOL_GPL(rhashtable_walk_peek);
-
 /**
  * rhashtable_walk_last_seen - Return the previously returned object, if available
  * @iter:	Hash table iterator
-- 
2.14.0.rc0.dirty


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