[PATCH 3/8] rhashtable: use cmpxchg() to protect ->future_tbl.

[NeilBrown]

Rather than borrowing one of the bucket locks to
protect ->future_tbl updates, use cmpxchg().
This gives more freedom to change how bucket locking
is implemented.

Signed-off-by: NeilBrown <neilb@xxxxxxxx>
---
 lib/rhashtable.c |   17 ++++++-----------
 1 file changed, 6 insertions(+), 11 deletions(-)

diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index 4a3f94e8e8a6..b73afe1dec7e 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -298,21 +298,16 @@ static int rhashtable_rehash_attach(struct rhashtable *ht,
 				    struct bucket_table *old_tbl,
 				    struct bucket_table *new_tbl)
 {
-	/* Protect future_tbl using the first bucket lock. */
-	spin_lock_bh(old_tbl->locks);
-
-	/* Did somebody beat us to it? */
-	if (rcu_access_pointer(old_tbl->future_tbl)) {
-		spin_unlock_bh(old_tbl->locks);
-		return -EEXIST;
-	}
-
 	/* Make insertions go into the new, empty table right away. Deletions
 	 * and lookups will be attempted in both tables until we synchronize.
+	 * The use of 'tmp' is simply to ensure we get the required memory
+	 * barriers before the cmpxchg().
 	 */
-	rcu_assign_pointer(old_tbl->future_tbl, new_tbl);
+	struct bucket_table *tmp;
 
-	spin_unlock_bh(old_tbl->locks);
+	rcu_assign_pointer(tmp, new_tbl);
+	if (cmpxchg(&old_tbl->future_tbl, NULL, tmp) != NULL)
+		return -EEXIST;
 
 	return 0;
 }