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 (file)
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -4629,6 +4629,13 @@ err_req:
        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;