RDS: Fix potential race around rds_i[bw]_allocation

"At rds_ib_recv_refill_one(), it first executes atomic_read(&rds_ib_allocation)
for if-condition checking,

and then executes atomic_inc(&rds_ib_allocation) if the condition was
not satisfied.

However, if any other code which updates rds_ib_allocation executes
between these two atomic operation executions,
it seems that it may result race condition. (especially when
rds_ib_allocation + 1 == rds_ib_sysctl_max_recv_allocation)"

This patch fixes this by using atomic_inc_unless to eliminate the
possibility of allocating more than rds_ib_sysctl_max_recv_allocation
and then decrementing the count if the allocation fails. It also
makes an identical change to the iwarp transport.

Reported-by: Shin Hong <hongshin@gmail.com>
Signed-off-by: Andy Grover <andy.grover@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/rds/ib_recv.c

@@ -143,15 +143,16 @@ static int rds_ib_recv_refill_one(struct rds_connection *conn,
 	int ret = -ENOMEM;
 
 	if (recv->r_ibinc == NULL) {
-		if (atomic_read(&rds_ib_allocation) >= rds_ib_sysctl_max_recv_allocation) {
+		if (!atomic_add_unless(&rds_ib_allocation, 1, rds_ib_sysctl_max_recv_allocation)) {
 			rds_ib_stats_inc(s_ib_rx_alloc_limit);
 			goto out;
 		}
 		recv->r_ibinc = kmem_cache_alloc(rds_ib_incoming_slab,
 						 kptr_gfp);
-		if (recv->r_ibinc == NULL)
+		if (recv->r_ibinc == NULL) {
+			atomic_dec(&rds_ib_allocation);
 			goto out;
-		atomic_inc(&rds_ib_allocation);
+		}
 		INIT_LIST_HEAD(&recv->r_ibinc->ii_frags);
 		rds_inc_init(&recv->r_ibinc->ii_inc, conn, conn->c_faddr);
 	}

net/rds/iw_recv.c

@@ -143,15 +143,16 @@ static int rds_iw_recv_refill_one(struct rds_connection *conn,
 	int ret = -ENOMEM;
 
 	if (recv->r_iwinc == NULL) {
-		if (atomic_read(&rds_iw_allocation) >= rds_iw_sysctl_max_recv_allocation) {
+		if (!atomic_add_unless(&rds_iw_allocation, 1, rds_iw_sysctl_max_recv_allocation)) {
 			rds_iw_stats_inc(s_iw_rx_alloc_limit);
 			goto out;
 		}
 		recv->r_iwinc = kmem_cache_alloc(rds_iw_incoming_slab,
 						 kptr_gfp);
-		if (recv->r_iwinc == NULL)
+		if (recv->r_iwinc == NULL) {
+			atomic_dec(&rds_iw_allocation);
 			goto out;
-		atomic_inc(&rds_iw_allocation);
+		}
 		INIT_LIST_HEAD(&recv->r_iwinc->ii_frags);
 		rds_inc_init(&recv->r_iwinc->ii_inc, conn, conn->c_faddr);
 	}