btrfs: add barrier for waitqueue_active in clear_btree_io_tree

waitqueue_active should be preceded by a barrier, in this function we
don't need to call it all the time.

Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 74bc333..e8e5b5a 100644 (file)
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -82,6 +82,12 @@ void btrfs_put_transaction(struct btrfs_transaction *transaction)
 static void clear_btree_io_tree(struct extent_io_tree *tree)
 {
        spin_lock(&tree->lock);
+       /*
+        * Do a single barrier for the waitqueue_active check here, the state
+        * of the waitqueue should not change once clear_btree_io_tree is
+        * called.
+        */
+       smp_mb();
        while (!RB_EMPTY_ROOT(&tree->state)) {
                struct rb_node *node;
                struct extent_state *state;