linux-kernel - Re: [PATCH 3/3] io_uring: move *queue_link

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Date:   Tue, 17 Dec 2019 15:00:57 +0100
From:   Dmitry Dolgov <9erthalion6@...il.com>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] io_uring: move *queue_link_head() from common path

> On Tue, Dec 17, 2019 at 02:22:09AM +0300, Pavel Begunkov wrote:
>
> Move io_queue_link_head() to links handling code in io_submit_sqe(),
> so it wouldn't need extra checks and would have better data locality.
>
> ---
>  fs/io_uring.c | 32 ++++++++++++++------------------
>  1 file changed, 14 insertions(+), 18 deletions(-)
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index bac9e711e38d..a880ed1409cb 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -3373,13 +3373,15 @@ static bool io_submit_sqe(struct io_kiocb *req, struct io_submit_state *state,
>  			  struct io_kiocb **link)
>  {
>  	struct io_ring_ctx *ctx = req->ctx;
> +	unsigned int sqe_flags;
>  	int ret;
>
> +	sqe_flags = READ_ONCE(req->sqe->flags);

Just out of curiosity, why READ_ONCE it necessary here? I though, that
since io_submit_sqes happens within a uring_lock, it's already
protected. Do I miss something?

> @@ -3421,9 +3423,15 @@ static bool io_submit_sqe(struct io_kiocb *req, struct io_submit_state *state,
>  		}
>  		trace_io_uring_link(ctx, req, head);
>  		list_add_tail(&req->link_list, &head->link_list);
> -	} else if (req->sqe->flags & (IOSQE_IO_LINK|IOSQE_IO_HARDLINK)) {
> +
> +		/* last request of a link, enqueue the link */
> +		if (!(sqe_flags & IOSQE_IO_LINK)) {

Yes, as you mentioned in the previous email, it seems correct that if
IOSQE_IO_HARDLINK imply IOSQE_IO_LINK, then here we need to check both.

> +			io_queue_link_head(head);
> +			*link = NULL;
> +		}
> +	} else if (sqe_flags & (IOSQE_IO_LINK|IOSQE_IO_HARDLINK)) {