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Subject: Re: [PATCH v2 2/2] io_uring: io_queue_link*() right after submit
To:     Pavel Begunkov <asml.silence@gmail.com>,
        Jens Axboe <axboe@kernel.dk>, io-uring@vger.kernel.org,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
References: <cover.1572988512.git.asml.silence@gmail.com>
 <85a316b577e1b5204d27a96a7ce452ed6be3c2ae.1572988512.git.asml.silence@gmail.com>
From:   Bob Liu <bob.liu@oracle.com>
Message-ID: <8700c9a3-01aa-2af6-c275-1f17734c2cc5@oracle.com>
Date:   Wed, 6 Nov 2019 16:36:40 +0800
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On 11/6/19 5:22 AM, Pavel Begunkov wrote:
> After a call to io_submit_sqe(), it's already known whether it needs
> to queue a link or not. Do it there, as it's simplier and doesn't keep
> an extra variable across the loop.
> 
> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> ---
>  fs/io_uring.c | 22 ++++++++++------------
>  1 file changed, 10 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index ebe2a4edd644..82c2da99cb5c 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -2687,7 +2687,6 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
>  	struct io_submit_state state, *statep = NULL;
>  	struct io_kiocb *link = NULL;
>  	struct io_kiocb *shadow_req = NULL;
> -	bool prev_was_link = false;
>  	int i, submitted = 0;
>  	bool mm_fault = false;
>  
> @@ -2710,17 +2709,6 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
>  			}
>  		}
>  
> -		/*
> -		 * If previous wasn't linked and we have a linked command,
> -		 * that's the end of the chain. Submit the previous link.
> -		 */
> -		if (!prev_was_link && link) {
> -			io_queue_link_head(ctx, link, &link->submit, shadow_req);
> -			link = NULL;
> -			shadow_req = NULL;
> -		}
> -		prev_was_link = (s.sqe->flags & IOSQE_IO_LINK) != 0;
> -
>  		if (link && (s.sqe->flags & IOSQE_IO_DRAIN)) {
>  			if (!shadow_req) {
>  				shadow_req = io_get_req(ctx, NULL);
> @@ -2741,6 +2729,16 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
>  		trace_io_uring_submit_sqe(ctx, s.sqe->user_data, true, async);
>  		io_submit_sqe(ctx, &s, statep, &link);
>  		submitted++;
> +
> +		/*
> +		 * If previous wasn't linked and we have a linked command,
> +		 * that's the end of the chain. Submit the previous link.
> +		 */
> +		if (!(s.sqe->flags & IOSQE_IO_LINK) && link) 
The behavior changed to 'current seq' instead of previous after dropping prev_was_link?

> +			io_queue_link_head(ctx, link, &link->submit, shadow_req);
> +			link = NULL;
> +			shadow_req = NULL;
> +		}
>  	}
>  
>  	if (link)
>