linux-kernel - Re: [PATCH 3/4] aio: implement IOCB_CMD

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Date:   Thu, 2 Aug 2018 11:00:52 +0200
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,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] aio: implement IOCB_CMD_POLL

On Thu, Aug 02, 2018 at 12:54:12AM +0100, Al Viro wrote:
> On Mon, Jul 30, 2018 at 09:15:43AM +0200, Christoph Hellwig wrote:
> 
> > +	apt.error = -EINVAL; /* same as no support for IOCB_CMD_POLL */
> 
> > +	mask = vfs_poll(req->file, &apt.pt) & req->events;
> > +	if (mask || apt.error) {
> > +		bool removed = false;
> > +
> > +		/* we did not manage to set up a waitqueue, done */
> > +		if (unlikely(!req->head))
> > +			goto out_fput;
> 
> > +out_fput:
> > +	fput(req->file);
> > +	return apt.error;
> 
> Ugh...  So anything that simply returns a constant value, without
> even bothering to do poll_wait() (on the theory that no matter how
> much you wait, nothing will change) is going to git -EINVAL?
> What am I missing here?

I can change it to return that constant value, but is it really going
to be useful to keep resubmitting an iocb for something that we can't
actually poll?  I don't think we help the application with that at all.