io_uring: add comment for drain_next

mainline inclusion
from mainline-5.6-rc1
commit 8cdf2193
category: feature
bugzilla: https://bugzilla.openeuler.org/show_bug.cgi?id=27
CVE: NA
---------------------------

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: Nyangerkun <yangerkun@huawei.com>
Reviewed-by: Nzhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: NCheng Jian <cj.chengjian@huawei.com>

fs/io_uring.c

@@ -4571,6 +4571,13 @@ 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;