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@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/fs/io_uring.c b/fs/io_uring.c
index 4bde5da..a700ee5 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -4629,6 +4629,13 @@ 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;
