Btrfs: use RCU instead of a spinlock to protect the root node

The pointer to the extent buffer for the root of each tree
is protected by a spinlock so that we can safely read the pointer
and take a reference on the extent buffer.

But now that the extent buffers are freed via RCU, we can safely
use rcu_read_lock instead.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 8 insertions, 19 deletions

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 73e53009e126..8680110f0a5a 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -147,10 +147,11 @@ noinline void btrfs_release_path(struct btrfs_root *root, struct btrfs_path *p)
 struct extent_buffer *btrfs_root_node(struct btrfs_root *root)
 {
         struct extent_buffer *eb;
-        spin_lock(&root->node_lock);
-        eb = root->node;
+
+        rcu_read_lock();
+        eb = rcu_dereference(root->node);
         extent_buffer_get(eb);
-        spin_unlock(&root->node_lock);
+        rcu_read_unlock();
         return eb;
 }
 
@@ -165,14 +166,8 @@ struct extent_buffer *btrfs_lock_root_node(struct btrfs_root *root)
         while (1) {
                 eb = btrfs_root_node(root);
                 btrfs_tree_lock(eb);
-
-                spin_lock(&root->node_lock);
-                if (eb == root->node) {
-                        spin_unlock(&root->node_lock);
+                if (eb == root->node)
                         break;
-                }
-                spin_unlock(&root->node_lock);
-
                 btrfs_tree_unlock(eb);
                 free_extent_buffer(eb);
         }
@@ -458,10 +453,8 @@ static noinline int __btrfs_cow_block(struct btrfs_trans_handle *trans,
                 else
                         parent_start = 0;
 
-                spin_lock(&root->node_lock);
-                root->node = cow;
                 extent_buffer_get(cow);
-                spin_unlock(&root->node_lock);
+                rcu_assign_pointer(root->node, cow);
 
                 btrfs_free_tree_block(trans, root, buf, parent_start,
                                       last_ref);
@@ -930,9 +923,7 @@ static noinline int balance_level(struct btrfs_trans_handle *trans,
                         goto enospc;
                 }
 
-                spin_lock(&root->node_lock);
-                root->node = child;
-                spin_unlock(&root->node_lock);
+                rcu_assign_pointer(root->node, child);
 
                 add_root_to_dirty_list(root);
                 btrfs_tree_unlock(child);
@@ -2007,10 +1998,8 @@ static noinline int insert_new_root(struct btrfs_trans_handle *trans,
 
         btrfs_mark_buffer_dirty(c);
 
-        spin_lock(&root->node_lock);
         old = root->node;
-        root->node = c;
-        spin_unlock(&root->node_lock);
+        rcu_assign_pointer(root->node, c);
 
         /* the super has an extra ref to root->node */
         free_extent_buffer(old);