io_uring: add comment for drain_next

to #26323588

commit 8cdf2193a3335b4cfb6e023b41ac293d0843d287 upstream.

Draining the middle of a link is tricky, so leave a comment there
Signed-off-by: NPavel Begunkov <asml.silence@gmail.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>
Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com>
Acked-by: NXiaoguang Wang <xiaoguang.wang@linux.alibaba.com>

fs/io_uring.c

@@ -4626,6 +4626,13 @@ static bool io_submit_sqe(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 	if (*link) {
 		struct io_kiocb *head = *link;
 
+		/*
+		 * Taking sequential execution of a link, draining both sides
+		 * of the link also fullfils IOSQE_IO_DRAIN semantics for all
+		 * requests in the link. So, it drains the head and the
+		 * next after the link request. The last one is done via
+		 * drain_next flag to persist the effect across calls.
+		 */
 		if (sqe_flags & IOSQE_IO_DRAIN) {
 			head->flags |= REQ_F_IO_DRAIN;
 			ctx->drain_next = 1;