linux-kernel - [PATCH 1/8] io_uring: add comment for drain

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Date:   Sat, 25 Jan 2020 00:40:24 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 1/8] io_uring: add comment for drain_next

Draining the middle of a link is tricky, so leave a comment there

Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
---
 fs/io_uring.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 25f29ef81698..c7b38e5f72a1 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -4702,6 +4702,13 @@ 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;
-- 
2.24.0