Btrfs: be more polite in the async caching threads

The semaphore used by the async caching threads can prevent a
transaction commit, which can make the FS appear to stall.  This
releases the semaphore more often when a transaction commit is
in progress.
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent-tree.c

@@ -302,10 +302,11 @@ again:
 			else if (ret)
 				break;
 
-			if (need_resched()) {
+			if (need_resched() ||
+			    btrfs_transaction_in_commit(fs_info)) {
 				btrfs_release_path(fs_info->extent_root, path);
 				up_read(&fs_info->extent_commit_sem);
-				cond_resched();
+				schedule_timeout(1);
 				goto again;
 			}
 

fs/btrfs/transaction.c

@@ -857,6 +857,16 @@ static void update_super_roots(struct btrfs_root *root)
 	super->root_level = root_item->level;
 }
 
+int btrfs_transaction_in_commit(struct btrfs_fs_info *info)
+{
+	int ret = 0;
+	spin_lock(&info->new_trans_lock);
+	if (info->running_transaction)
+		ret = info->running_transaction->in_commit;
+	spin_unlock(&info->new_trans_lock);
+	return ret;
+}
+
 int btrfs_commit_transaction(struct btrfs_trans_handle *trans,
 			     struct btrfs_root *root)
 {

fs/btrfs/transaction.h

@@ -107,4 +107,5 @@ int btrfs_record_root_in_trans(struct btrfs_trans_handle *trans,
 				struct btrfs_root *root);
 int btrfs_write_and_wait_marked_extents(struct btrfs_root *root,
 					struct extent_io_tree *dirty_pages);
+int btrfs_transaction_in_commit(struct btrfs_fs_info *info);
 #endif