[PATCH RFC POC 24/50] file: make FD_ADD() a standalone immediate install

[Christian Brauner <brauner@kernel.org>]

FD_ADD() was built on FD_PREPARE() + fd_publish(). Give it its own small
macro that allocates the descriptor so it stays an immediate one-shot
for the hot descriptor-is-the-result paths (open, socket, accept, memfd
etc.).

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 include/linux/file.h | 36 +++++++++++++++++++++---------------
 1 file changed, 21 insertions(+), 15 deletions(-)

diff --git a/include/linux/file.h b/include/linux/file.h
index 04dd85cdd9bf..1ee7f058a882 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -259,24 +259,30 @@ static inline int class_fd_prepare_lock_err(const struct fd_prepare *fdf)
 		take_fd(fdp->__fd);                            \
 	})
 
-/* Do not use directly. */
-#define __FD_ADD(_fdf, _fd_flags, _file_owned)            \
-	({                                                \
-		FD_PREPARE(_fdf, _fd_flags, _file_owned); \
-		s32 ret = _fdf.err;                       \
-		if (likely(!ret))                         \
-			ret = fd_publish(_fdf);           \
-		ret;                                      \
-	})
-
 /*
- * FD_ADD - Allocate and install an fd and file in one step.
+ * FD_ADD - allocate a descriptor, build the file and install it in one step.
  * @_fd_flags: flags for get_unused_fd_flags()
- * @_file_owned: struct file to take ownership of
+ * @_file_owned: struct file to take ownership of (can be an expression)
  *
- * Returns the allocated fd number, or negative error code on failure.
+ * The file expression is evaluated only after the descriptor is allocated, so
+ * a full table does not run its side effects. Drops the file and returns a
+ * negative errno on failure. Installs immediately: for anything more than a
+ * bare install-and-return (reporting the number, configuring the file,
+ * installing several descriptors) use fd_prepare()/fd_stage().
  */
-#define FD_ADD(_fd_flags, _file_owned) \
-	__FD_ADD(__UNIQUE_ID(fd_prepare), _fd_flags, _file_owned)
+#define FD_ADD(_fd_flags, _file_owned)					\
+({									\
+	int __fd = get_unused_fd_flags(_fd_flags);			\
+	if (likely(__fd >= 0)) {					\
+		struct file *__file = (_file_owned);			\
+		if (unlikely(IS_ERR_OR_NULL(__file))) {			\
+			put_unused_fd(__fd);				\
+			__fd = __file ? PTR_ERR(__file) : -ENOMEM;	\
+		} else {						\
+			fd_install(__fd, __file);			\
+		}							\
+	}								\
+	__fd;								\
+})
 
 #endif /* __LINUX_FILE_H */

-- 
2.53.0