rhashtable: Fix reader/rehash race

There is a potential race condition between readers and the rehasher.
In particular, the rehasher could have started a rehash while the
reader finishes a scan of the old table but fails to see the new
table pointer.

This patch closes this window by adding smp_wmb/smp_rmb.

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

1 files changed, 6 insertions, 0 deletions

diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index 6ffc793145f3..68210cc2bab8 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -271,6 +271,9 @@ static void rhashtable_rehash(struct rhashtable *ht,
          */
         rcu_assign_pointer(ht->future_tbl, new_tbl);
 
+        /* Ensure the new table is visible to readers. */
+        smp_wmb();
+
         for (old_hash = 0; old_hash < old_tbl->size; old_hash++)
                 rhashtable_rehash_chain(ht, old_hash);
 
@@ -618,6 +621,9 @@ restart:
                 return rht_obj(ht, he);
         }
 
+        /* Ensure we see any new tables. */
+        smp_rmb();
+
         old_tbl = tbl;
         tbl = rht_dereference_rcu(ht->future_tbl, ht);
         if (unlikely(tbl != old_tbl))