[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 <ne...@suse.com>
---
 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;
 }