io_wq: add per-wq work handler instead of per work

io_uring is the only user of io-wq, and now it uses only io-wq callback
for all its requests, namely io_wq_submit_work(). Instead of storing
work->runner callback in each instance of io_wq_work, keep it in io-wq
itself.

pros:
- reduces io_wq_work size
- more robust -- ->func won't be invalidated with mem{cpy,set}(req)
- helps other work

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io-wq.h

@@ -85,7 +85,6 @@ static inline void wq_list_del(struct io_wq_work_list *list,
 
 struct io_wq_work {
 	struct io_wq_work_node list;
-	void (*func)(struct io_wq_work **);
 	struct files_struct *files;
 	struct mm_struct *mm;
 	const struct cred *creds;
@@ -94,9 +93,9 @@ struct io_wq_work {
 	pid_t task_pid;
 };
 
-#define INIT_IO_WORK(work, _func)				\
+#define INIT_IO_WORK(work)					\
 	do {							\
-		*(work) = (struct io_wq_work){ .func = _func };	\
+		*(work) = (struct io_wq_work){};		\
 	} while (0)						\
 
 static inline struct io_wq_work *wq_next_work(struct io_wq_work *work)
@@ -108,10 +107,12 @@ static inline struct io_wq_work *wq_next_work(struct io_wq_work *work)
 }
 
 typedef void (free_work_fn)(struct io_wq_work *);
+typedef void (io_wq_work_fn)(struct io_wq_work **);
 
 struct io_wq_data {
 	struct user_struct *user;
 
+	io_wq_work_fn *do_work;
 	free_work_fn *free_work;
 };