btrfs: provide simple rcu-walk ACL implementation

This simple implementation just checks for no ACLs on the inode, and
if so, then the rcu-walk may proceed, otherwise fail it.
Signed-off-by: Nick Piggin <npiggin@kernel.dk>

fs/btrfs/acl.c

@@ -187,12 +187,14 @@ static int btrfs_xattr_acl_set(struct dentry *dentry, const char *name,
 
 int btrfs_check_acl(struct inode *inode, int mask, unsigned int flags)
 {
-	struct posix_acl *acl;
 	int error = -EAGAIN;
 
-	if (flags & IPERM_FLAG_RCU)
-		return -ECHILD;
+	if (flags & IPERM_FLAG_RCU) {
+		if (!negative_cached_acl(inode, ACL_TYPE_ACCESS))
+			error = -ECHILD;
 
+	} else {
+		struct posix_acl *acl;
 		acl = btrfs_get_acl(inode, ACL_TYPE_ACCESS);
 		if (IS_ERR(acl))
 			return PTR_ERR(acl);
@@ -200,6 +202,7 @@ int btrfs_check_acl(struct inode *inode, int mask, unsigned int flags)
 			error = posix_acl_permission(inode, acl, mask);
 			posix_acl_release(acl);
 		}
+	}
 
 	return error;
 }

fs/btrfs/inode.c

@@ -7213,9 +7213,6 @@ static int btrfs_set_page_dirty(struct page *page)
 
 static int btrfs_permission(struct inode *inode, int mask, unsigned int flags)
 {
-	if (flags & IPERM_FLAG_RCU)
-		return -ECHILD;
-
 	if ((BTRFS_I(inode)->flags & BTRFS_INODE_READONLY) && (mask & MAY_WRITE))
 		return -EACCES;
 	return generic_permission(inode, mask, flags, btrfs_check_acl);