rds: rcu-ize rds_ib_get_device()

rds_ib_get_device is called very often as we turn an
ip address into a corresponding device structure.  It currently
take a global spinlock as it walks different lists to find active
devices.

This commit changes the lists over to RCU, which isn't very complex
because they are not updated very often at all.
Signed-off-by: Chris Mason <chris.mason@oracle.com>

net/rds/ib.c

@@ -137,6 +137,7 @@ void rds_ib_remove_one(struct ib_device *device)
 	if (!rds_ibdev)
 		return;
 
+	synchronize_rcu();
 	list_for_each_entry_safe(i_ipaddr, i_next, &rds_ibdev->ipaddr_list, list) {
 		list_del(&i_ipaddr->list);
 		kfree(i_ipaddr);

net/rds/ib_rdma.c

@@ -32,6 +32,7 @@
  */
 #include <linux/kernel.h>
 #include <linux/slab.h>
+#include <linux/rculist.h>
 
 #include "rds.h"
 #include "ib.h"
@@ -83,14 +84,14 @@ static struct rds_ib_device *rds_ib_get_device(__be32 ipaddr)
 	struct rds_ib_ipaddr *i_ipaddr;
 
 	list_for_each_entry(rds_ibdev, &rds_ib_devices, list) {
-		spin_lock_irq(&rds_ibdev->spinlock);
-		list_for_each_entry(i_ipaddr, &rds_ibdev->ipaddr_list, list) {
+		rcu_read_lock();
+		list_for_each_entry_rcu(i_ipaddr, &rds_ibdev->ipaddr_list, list) {
 			if (i_ipaddr->ipaddr == ipaddr) {
-				spin_unlock_irq(&rds_ibdev->spinlock);
+				rcu_read_unlock();
 				return rds_ibdev;
 			}
 		}
-		spin_unlock_irq(&rds_ibdev->spinlock);
+		rcu_read_unlock();
 	}
 
 	return NULL;
@@ -107,7 +108,7 @@ static int rds_ib_add_ipaddr(struct rds_ib_device *rds_ibdev, __be32 ipaddr)
 	i_ipaddr->ipaddr = ipaddr;
 
 	spin_lock_irq(&rds_ibdev->spinlock);
-	list_add_tail(&i_ipaddr->list, &rds_ibdev->ipaddr_list);
+	list_add_tail_rcu(&i_ipaddr->list, &rds_ibdev->ipaddr_list);
 	spin_unlock_irq(&rds_ibdev->spinlock);
 
 	return 0;
@@ -116,16 +117,23 @@ static int rds_ib_add_ipaddr(struct rds_ib_device *rds_ibdev, __be32 ipaddr)
 static void rds_ib_remove_ipaddr(struct rds_ib_device *rds_ibdev, __be32 ipaddr)
 {
 	struct rds_ib_ipaddr *i_ipaddr, *next;
+	struct rds_ib_ipaddr *to_free = NULL;
+
 
 	spin_lock_irq(&rds_ibdev->spinlock);
-	list_for_each_entry_safe(i_ipaddr, next, &rds_ibdev->ipaddr_list, list) {
+	list_for_each_entry_rcu(i_ipaddr, &rds_ibdev->ipaddr_list, list) {
 		if (i_ipaddr->ipaddr == ipaddr) {
-			list_del(&i_ipaddr->list);
-			kfree(i_ipaddr);
+			list_del_rcu(&i_ipaddr->list);
+			to_free = i_ipaddr;
 			break;
 		}
 	}
 	spin_unlock_irq(&rds_ibdev->spinlock);
+
+	if (to_free) {
+		synchronize_rcu();
+		kfree(to_free);
+	}
 }
 
 int rds_ib_update_ipaddr(struct rds_ib_device *rds_ibdev, __be32 ipaddr)