netdev - Re: [PATCH v2 1/1] eventfd: implementation of EFD

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Date:	Thu, 14 Feb 2013 21:24:11 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Martin Sustrik <sustrik@...bpm.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Sha Zhengju <handai.szj@...bao.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, Michael Kerrisk <mtk.manpages@...il.com>,
	Davide Libenzi <davidel@...ilserver.org>,
	Andy Lutomirski <luto@...capital.net>,
	Eric Wong <normalperson@...t.net>
Subject: Re: [PATCH v2 1/1] eventfd: implementation of EFD_MASK flag

On Fri, 15 Feb 2013 04:42:27 +0100 Martin Sustrik <sustrik@...bpm.com> wrote:

> > This is a non-back-compatible userspace interface change.  A procfs
> > file which previously displayed
> >
> > 	eventfd-count: nnnn
> >
> > can now also display
> >
> > 	eventfd-mask: nnnn
> >
> > So existing userspace could misbehave.
> >
> > Please fully describe the proposed interface change in the changelog.
> > That description should include the full pathname of the procfs file
> > and example before-and-after output and a discussion of whether and why
> > the risk to existing userspace is acceptable.
> 
> I am not sure what the policy is here. Is not printing out the state of 
> the object acceptable way to maintain backward compatibility? If not so, 
> does new type of object require new procfs file, which, AFAIU, is the 
> only way to retain full backward compatibility?

Adding a new file is the only way I can think of to preserve the API. 
But from Andy's comment is sounds like we don't have to worry a lot
about back-compatibility.

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