rhashtable: Introduce rhashtable_walk_*

Some existing rhashtable users get too intimate with it by walking
the buckets directly.  This prevents us from easily changing the
internals of rhashtable.

This patch adds the helpers rhashtable_walk_init/exit/start/next/stop
which will replace these custom walkers.

They are meant to be usable for both procfs seq_file walks as well
as walking by a netlink dump.  The iterator structure should fit
inside a netlink dump cb structure, with at least one element to
spare.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 198 insertions, 0 deletions

diff --git a/include/linux/rhashtable.h b/include/linux/rhashtable.h
index e0337844358e..58851275fed9 100644
--- a/include/linux/rhashtable.h
+++ b/include/linux/rhashtable.h
@@ -18,6 +18,7 @@
 #ifndef _LINUX_RHASHTABLE_H
 #define _LINUX_RHASHTABLE_H
 
+#include <linux/compiler.h>
 #include <linux/list_nulls.h>
 #include <linux/workqueue.h>
 #include <linux/mutex.h>
@@ -111,6 +112,7 @@ struct rhashtable_params {
  * @p: Configuration parameters
  * @run_work: Deferred worker to expand/shrink asynchronously
  * @mutex: Mutex to protect current/future table swapping
+ * @walkers: List of active walkers
  * @being_destroyed: True if table is set up for destruction
  */
 struct rhashtable {
@@ -121,9 +123,36 @@ struct rhashtable {
         struct rhashtable_params        p;
         struct work_struct              run_work;
         struct mutex                    mutex;
+        struct list_head                walkers;
         bool                            being_destroyed;
 };
 
+/**
+ * struct rhashtable_walker - Hash table walker
+ * @list: List entry on list of walkers
+ * @resize: Resize event occured
+ */
+struct rhashtable_walker {
+        struct list_head list;
+        bool resize;
+};
+
+/**
+ * struct rhashtable_iter - Hash table iterator, fits into netlink cb
+ * @ht: Table to iterate through
+ * @p: Current pointer
+ * @walker: Associated rhashtable walker
+ * @slot: Current slot
+ * @skip: Number of entries to skip in slot
+ */
+struct rhashtable_iter {
+        struct rhashtable *ht;
+        struct rhash_head *p;
+        struct rhashtable_walker *walker;
+        unsigned int slot;
+        unsigned int skip;
+};
+
 static inline unsigned long rht_marker(const struct rhashtable *ht, u32 hash)
 {
         return NULLS_MARKER(ht->p.nulls_base + hash);
@@ -179,6 +208,12 @@ bool rhashtable_lookup_compare_insert(struct rhashtable *ht,
                                       bool (*compare)(void *, void *),
                                       void *arg);
 
+int rhashtable_walk_init(struct rhashtable *ht, struct rhashtable_iter *iter);
+void rhashtable_walk_exit(struct rhashtable_iter *iter);
+int rhashtable_walk_start(struct rhashtable_iter *iter) __acquires(RCU);
+void *rhashtable_walk_next(struct rhashtable_iter *iter);
+void rhashtable_walk_stop(struct rhashtable_iter *iter) __releases(RCU);
+
 void rhashtable_destroy(struct rhashtable *ht);
 
 #define rht_dereference(p, ht) \
diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index 904b419b72f5..057919164e23 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -484,6 +484,7 @@ static void rht_deferred_worker(struct work_struct *work)
 {
         struct rhashtable *ht;
         struct bucket_table *tbl;
+        struct rhashtable_walker *walker;
 
         ht = container_of(work, struct rhashtable, run_work);
         mutex_lock(&ht->mutex);
@@ -492,6 +493,9 @@ static void rht_deferred_worker(struct work_struct *work)
 
         tbl = rht_dereference(ht->tbl, ht);
 
+        list_for_each_entry(walker, &ht->walkers, list)
+                walker->resize = true;
+
         if (ht->p.grow_decision && ht->p.grow_decision(ht, tbl->size))
                 rhashtable_expand(ht);
         else if (ht->p.shrink_decision && ht->p.shrink_decision(ht, tbl->size))
@@ -822,6 +826,164 @@ exit:
 }
 EXPORT_SYMBOL_GPL(rhashtable_lookup_compare_insert);
 
+/**
+ * rhashtable_walk_init - Initialise an iterator
+ * @ht:         Table to walk over
+ * @iter:       Hash table Iterator
+ *
+ * This function prepares a hash table walk.
+ *
+ * Note that if you restart a walk after rhashtable_walk_stop you
+ * may see the same object twice.  Also, you may miss objects if
+ * there are removals in between rhashtable_walk_stop and the next
+ * call to rhashtable_walk_start.
+ *
+ * For a completely stable walk you should construct your own data
+ * structure outside the hash table.
+ *
+ * This function may sleep so you must not call it from interrupt
+ * context or with spin locks held.
+ *
+ * You must call rhashtable_walk_exit if this function returns
+ * successfully.
+ */
+int rhashtable_walk_init(struct rhashtable *ht, struct rhashtable_iter *iter)
+{
+        iter->ht = ht;
+        iter->p = NULL;
+        iter->slot = 0;
+        iter->skip = 0;
+
+        iter->walker = kmalloc(sizeof(*iter->walker), GFP_KERNEL);
+        if (!iter->walker)
+                return -ENOMEM;
+
+        mutex_lock(&ht->mutex);
+        list_add(&iter->walker->list, &ht->walkers);
+        mutex_unlock(&ht->mutex);
+
+        return 0;
+}
+EXPORT_SYMBOL_GPL(rhashtable_walk_init);
+
+/**
+ * rhashtable_walk_exit - Free an iterator
+ * @iter:       Hash table Iterator
+ *
+ * This function frees resources allocated by rhashtable_walk_init.
+ */
+void rhashtable_walk_exit(struct rhashtable_iter *iter)
+{
+        mutex_lock(&iter->ht->mutex);
+        list_del(&iter->walker->list);
+        mutex_unlock(&iter->ht->mutex);
+        kfree(iter->walker);
+}
+EXPORT_SYMBOL_GPL(rhashtable_walk_exit);
+
+/**
+ * rhashtable_walk_start - Start a hash table walk
+ * @iter:       Hash table iterator
+ *
+ * Start a hash table walk.  Note that we take the RCU lock in all
+ * cases including when we return an error.  So you must always call
+ * rhashtable_walk_stop to clean up.
+ *
+ * Returns zero if successful.
+ *
+ * Returns -EAGAIN if resize event occured.  Note that the iterator
+ * will rewind back to the beginning and you may use it immediately
+ * by calling rhashtable_walk_next.
+ */
+int rhashtable_walk_start(struct rhashtable_iter *iter)
+{
+        rcu_read_lock();
+
+        if (iter->walker->resize) {
+                iter->slot = 0;
+                iter->skip = 0;
+                iter->walker->resize = false;
+                return -EAGAIN;
+        }
+
+        return 0;
+}
+EXPORT_SYMBOL_GPL(rhashtable_walk_start);
+
+/**
+ * rhashtable_walk_next - Return the next object and advance the iterator
+ * @iter:       Hash table iterator
+ *
+ * Note that you must call rhashtable_walk_stop when you are finished
+ * with the walk.
+ *
+ * Returns the next object or NULL when the end of the table is reached.
+ *
+ * Returns -EAGAIN if resize event occured.  Note that the iterator
+ * will rewind back to the beginning and you may continue to use it.
+ */
+void *rhashtable_walk_next(struct rhashtable_iter *iter)
+{
+        const struct bucket_table *tbl;
+        struct rhashtable *ht = iter->ht;
+        struct rhash_head *p = iter->p;
+        void *obj = NULL;
+
+        tbl = rht_dereference_rcu(ht->tbl, ht);
+
+        if (p) {
+                p = rht_dereference_bucket_rcu(p->next, tbl, iter->slot);
+                goto next;
+        }
+
+        for (; iter->slot < tbl->size; iter->slot++) {
+                int skip = iter->skip;
+
+                rht_for_each_rcu(p, tbl, iter->slot) {
+                        if (!skip)
+                                break;
+                        skip--;
+                }
+
+next:
+                if (!rht_is_a_nulls(p)) {
+                        iter->skip++;
+                        iter->p = p;
+                        obj = rht_obj(ht, p);
+                        goto out;
+                }
+
+                iter->skip = 0;
+        }
+
+        iter->p = NULL;
+
+out:
+        if (iter->walker->resize) {
+                iter->p = NULL;
+                iter->slot = 0;
+                iter->skip = 0;
+                iter->walker->resize = false;
+                return ERR_PTR(-EAGAIN);
+        }
+
+        return obj;
+}
+EXPORT_SYMBOL_GPL(rhashtable_walk_next);
+
+/**
+ * rhashtable_walk_stop - Finish a hash table walk
+ * @iter:       Hash table iterator
+ *
+ * Finish a hash table walk.
+ */
+void rhashtable_walk_stop(struct rhashtable_iter *iter)
+{
+        rcu_read_unlock();
+        iter->p = NULL;
+}
+EXPORT_SYMBOL_GPL(rhashtable_walk_stop);
+
 static size_t rounded_hashtable_size(struct rhashtable_params *params)
 {
         return max(roundup_pow_of_two(params->nelem_hint * 4 / 3),
@@ -894,6 +1056,7 @@ int rhashtable_init(struct rhashtable *ht, struct rhashtable_params *params)
         memset(ht, 0, sizeof(*ht));
         mutex_init(&ht->mutex);
         memcpy(&ht->p, params, sizeof(*params));
+        INIT_LIST_HEAD(&ht->walkers);
 
         if (params->locks_mul)
                 ht->p.locks_mul = roundup_pow_of_two(params->locks_mul);