linux-kernel - Re: [PATCH 2/2] aio: propogate post-EIOCBQUEUED errors to completion event

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Date:	Mon, 19 Feb 2007 17:48:14 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Zach Brown <zach.brown@...cle.com>
Cc:	linux-aio@...ck.org, linux-kernel@...r.kernel.org,
	Benjamin LaHaise <bcrl@...ck.org>,
	Suparna bhattacharya <suparna@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Leonid Ananiev <leonid.i.ananiev@...el.com>
Subject: Re: [PATCH 2/2] aio: propogate post-EIOCBQUEUED errors to completion event

==> On Mon, 19 Feb 2007 13:38:35 -0800 (PST), Zach Brown <zach.brown@...cle.com> said:

Zach> This addresses an oops reported by Leonid Ananiev <leonid.i.ananiev@...el.com>
Zach> as archived at http://lkml.org/lkml/2007/2/8/337.
[snip]
Zach> So this patch introduces a helper, aio_propogate_error(), 

...which is spelled incorrectly:  aio_propagate_error.

Zach> which queues post-submission errors in the iocb so that they are
Zach> given to the user completion event when aio_complete() is
Zach> finally called.

Ugly, but I can't think of a better way to do it, either.

> +
> +	if (kiocbIsInserted(req))
> +		aio_ring_insert_entry(ctx, req);
> +

This confused me at first reading.  If it's inserted, then insert it.
Perhaps KIF_COMPLETED would be a better name?

> +/*
> + * This function is used to make sure that an error is communicated to
> + * userspace on iocb completion without stopping -EIOCBQUEUED from bubbling up
> + * to fs/aio.c from the place where it originated.
> + *
> + * If we have an existing -EIOCBQUEUED it must be returned all the way to
> + * fs/aio.c so that it doesn't double-complete the iocb along with whoever
> + * returned -EIOCBQUEUED..  In that case we put the new error in the iocb.  It
> + * will be returned to userspace *intead of* the first result code given to
> + * aio_complete().  Use this only for errors which must overwrite whatever the
> + * return code might have been.  The first non-zero new_err given to this
> + * function for a given iocb will be returned to userspace.
> + */
> +static inline int aio_propogate_error(struct kiocb *iocb, int existing_err,
> +				      int new_err)
> +{
> +	if (existing_err != -EIOCBQUEUED)
> +		return new_err;
> +	if (!iocb->ki_pending_err)
> +		iocb->ki_pending_err = new_err;
> +	return -EIOCBQUEUED;
> +}
> +

I think that we would ideally cancel the I/Os in flight since we are
going to throw away the results.  However, we don't exactly support
cancellation today.  So, I have no objection to this less optimal
solution.

-Jeff
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