linux-kernel - Re: [PATCH 2/2] aio: make aio_read_events_ring be aware of aio

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Message-ID: <20140228205243.GA7325@kmo-pixel>
Date:	Fri, 28 Feb 2014 12:52:43 -0800
From:	Kent Overstreet <kmo@...erainc.com>
To:	Gu Zheng <guz.fnst@...fujitsu.com>
Cc:	Benjamin <bcrl@...ck.org>, Jens <axboe@...nel.dk>,
	linux-aio@...ck.org, linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] aio: make aio_read_events_ring be aware of
 aio_complete

On Fri, Feb 28, 2014 at 06:27:18PM +0800, Gu Zheng wrote:
> Commit 5ffac122dbda8(aio: Don't use ctx->tail unnecessarily) uses
> ring->tail rather than the ctx->tail, but with this change, we fetch 'tail'
> only once at the start, so that we can not be aware of adding event by aio_complete
> when reading events. It seems a regression.

"Seems a regression" is not good enough. What breaks?

> So here we fetch the ring->tail in start of the loop each time to make it be
> aware of adding event from aio_complete.
> 
> Signed-off-by: Gu Zheng <guz.fnst@...fujitsu.com>
> ---
>  fs/aio.c |    6 +++++-
>  1 files changed, 5 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/aio.c b/fs/aio.c
> index 7eaa631..f5b8551 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -1029,10 +1029,14 @@ static long aio_read_events_ring(struct kioctx *ctx,
>  		struct io_event *ev;
>  		struct page *page;
>  
> -		avail = (head <= tail ?  tail : ctx->nr_events) - head;
> +		ring = kmap_atomic(ctx->ring_pages[0]);
> +		tail = ring->tail;
> +		kunmap_atomic(ring);
> +
>  		if (head == tail)
>  			break;
>  
> +		avail = (head <= tail ?  tail : ctx->nr_events) - head;
>  		avail = min(avail, nr - ret);
>  		avail = min_t(long, avail, AIO_EVENTS_PER_PAGE -
>  			    ((head + AIO_EVENTS_OFFSET) % AIO_EVENTS_PER_PAGE));
> -- 
> 1.7.7
> 
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