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

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