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

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Message-ID: <CALCETrWxLiesdRc6v11uaSVnQYXJQwP+gR7reG3x-iU44wV2HA@mail.gmail.com>
Date:	Fri, 15 Feb 2013 09:32:37 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Martin Sustrik <sustrik@...bpm.com>,
	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>,
	Eric Wong <normalperson@...t.net>
Subject: Re: [PATCH v2 1/1] eventfd: implementation of EFD_MASK flag

On Thu, Feb 14, 2013 at 9:24 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> 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.
>

I'm not even convinced there's an issue in the first place (other than
the fact that use of this feature will break old criu, regardless of
/proc changes).  The fdinfo files already vary by descriptor type.
Anything that screws up if unexpected fields are present is already
screwed.

--Andy
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