io_uring: track link's head and tail during submit

Explicitly save not only a link's head in io_submit_sqe[s]() but the
tail as well. That's in preparation for keeping linked requests in a
singly linked list.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -6536,8 +6536,13 @@ static inline void io_queue_link_head(struct io_kiocb *req,
 		io_queue_sqe(req, NULL, cs);
 }
 
+struct io_submit_link {
+	struct io_kiocb *head;
+	struct io_kiocb *last;
+};
+
 static int io_submit_sqe(struct io_kiocb *req, const struct io_uring_sqe *sqe,
-			 struct io_kiocb **link, struct io_comp_state *cs)
+			 struct io_submit_link *link, struct io_comp_state *cs)
 {
 	struct io_ring_ctx *ctx = req->ctx;
 	int ret;
@@ -6549,8 +6554,8 @@ static int io_submit_sqe(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 	 * submitted sync once the chain is complete. If none of those
 	 * conditions are true (normal request), then just queue it.
 	 */
-	if (*link) {
-		struct io_kiocb *head = *link;
+	if (link->head) {
+		struct io_kiocb *head = link->head;
 
 		/*
 		 * Taking sequential execution of a link, draining both sides
@@ -6571,11 +6576,12 @@ static int io_submit_sqe(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 		}
 		trace_io_uring_link(ctx, req, head);
 		list_add_tail(&req->link_list, &head->link_list);
+		link->last = req;
 
 		/* last request of a link, enqueue the link */
 		if (!(req->flags & (REQ_F_LINK | REQ_F_HARDLINK))) {
 			io_queue_link_head(head, cs);
-			*link = NULL;
+			link->head = NULL;
 		}
 	} else {
 		if (unlikely(ctx->drain_next)) {
@@ -6589,7 +6595,8 @@ static int io_submit_sqe(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 			ret = io_req_defer_prep(req, sqe);
 			if (unlikely(ret))
 				req->flags |= REQ_F_FAIL_LINK;
-			*link = req;
+			link->head = req;
+			link->last = req;
 		} else {
 			io_queue_sqe(req, sqe, cs);
 		}
@@ -6769,7 +6776,7 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
 static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr)
 {
 	struct io_submit_state state;
-	struct io_kiocb *link = NULL;
+	struct io_submit_link link;
 	int i, submitted = 0;
 
 	/* if we have a backlog and couldn't flush it all, return BUSY */
@@ -6789,6 +6796,7 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr)
 	refcount_add(nr, &current->usage);
 
 	io_submit_state_start(&state, ctx, nr);
+	link.head = NULL;
 
 	for (i = 0; i < nr; i++) {
 		const struct io_uring_sqe *sqe;
@@ -6834,8 +6842,8 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr)
 		percpu_counter_sub(&tctx->inflight, unused);
 		put_task_struct_many(current, unused);
 	}
-	if (link)
-		io_queue_link_head(link, &state.comp);
+	if (link.head)
+		io_queue_link_head(link.head, &state.comp);
 	io_submit_state_end(&state);
 
 	 /* Commit SQ ring head once we've consumed and submitted all SQEs */