From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1161091Ab3BODmc (ORCPT <rfc822;w@1wt.eu>);
	Thu, 14 Feb 2013 22:42:32 -0500
Received: from chrocht.moloch.sk ([62.176.169.44]:60199 "EHLO mail.moloch.sk"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S932322Ab3BODmb (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Thu, 14 Feb 2013 22:42:31 -0500
Message-ID: <511DAEA3.4080201@250bpm.com>
Date: Fri, 15 Feb 2013 04:42:27 +0100
From: Martin Sustrik <sustrik@250bpm.com>
User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.24) Gecko/20111109 Thunderbird/3.1.16
MIME-Version: 1.0
To: Andrew Morton <akpm@linux-foundation.org>
CC: Alexander Viro <viro@zeniv.linux.org.uk>,
        Sha Zhengju <handai.szj@taobao.com>, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Davide Libenzi <davidel@xmailserver.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Eric Wong <normalperson@yhbt.net>
Subject: Re: [PATCH v2 1/1] eventfd: implementation of EFD_MASK flag
References: <1360311077-14474-1-git-send-email-sustrik@250bpm.com> <20130214145430.04f8750c.akpm@linux-foundation.org>
In-Reply-To: <20130214145430.04f8750c.akpm@linux-foundation.org>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

Hi Andrew,

Thanks for the detailed code review! I'll have a look at all the 
problems you've pointed out, however, one quick question:

>> -	ret = seq_printf(m, "eventfd-count: %16llx\n",
>> -			 (unsigned long long)ctx->count);
>> +	if (ctx->flags&  EFD_MASK) {
>> +		ret = seq_printf(m, "eventfd-mask: %x\n",
>> +				 (unsigned)ctx->mask.events);
>> +	} else {
>> +		ret = seq_printf(m, "eventfd-count: %16llx\n",
>> +				 (unsigned long long)ctx->count);
>> +	}
>>   	spin_unlock_irq(&ctx->wqh.lock);
>
> 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?

Martin