netdev - Re: [PATCH 06/28] aio: implement IOCB_CMD

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Message-ID: <20180323180520.GA7837@lst.de>
Date:   Fri, 23 Mar 2018 19:05:20 +0100
From:   Christoph Hellwig <hch@....de>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     Christoph Hellwig <hch@....de>, Avi Kivity <avi@...lladb.com>,
        linux-aio@...ck.org, linux-fsdevel@...r.kernel.org,
        netdev@...r.kernel.org, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/28] aio: implement IOCB_CMD_POLL

On Thu, Mar 22, 2018 at 06:16:53PM +0000, Al Viro wrote:
> On Thu, Mar 22, 2018 at 06:24:10PM +0100, Christoph Hellwig wrote:
> 
> > -static void aio_complete(struct aio_kiocb *iocb, long res, long res2)
> > +static bool aio_complete(struct aio_kiocb *iocb, long res, long res2,
> > +		unsigned complete_flags)
> 
> Looks like all callers are following that with "if returned true,
> fput(something)".  Does it really make any sense to keep that struct
> file * in different fields?

struct kiocb is used not just for aio, but for our normal read/write_iter
APIs, and it is not suitable for poll or fsync.  So I can't really find
a good way to keep it common except for duplicating it in struct kiocb
and strut aio_iocb.  But maybe we could pass a struct file argument
to aio_complete().

> Wait a sec...  What ordering do we want for
> 	* call(s) of ->ki_complete
> 	* call (if any) of ->ki_cancel
> 	* dropping reference to struct file
> and what are the expected call chains for all of those?

fput must be done exactly once from inside ->ki_complete OR ->ki_cancel
in case it did manage to do the actual completion.  Reference to struct
file isn't needed in aio_complete, but if aio_complete decided who
won the race we'll have to put after it (or inside it if we want to make
it common)