From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner+w=401wt.eu-S1757134AbZEOWPS@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1757134AbZEOWPS (ORCPT <rfc822;w@1wt.eu>);
	Fri, 15 May 2009 18:15:18 -0400
Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754216AbZEOWO7
	(ORCPT <rfc822;linux-kernel-outgoing>);
	Fri, 15 May 2009 18:14:59 -0400
Received: from mail-fx0-f158.google.com ([209.85.220.158]:64615 "EHLO
	mail-fx0-f158.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1754214AbZEOWO5 (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 15 May 2009 18:14:57 -0400
DomainKey-Signature: a=rsa-sha1; c=nofws;
        d=gmail.com; s=gamma;
        h=message-id:date:from:user-agent:mime-version:to:cc:subject
         :references:in-reply-to:x-enigmail-version:content-type
         :content-transfer-encoding;
        b=xLpdweJL+Dml3GnYYkBRPWqW3JFuOxr0hmr++LG5i/wIBaZX5GvboQVeoVec4/SRVe
         3UQ7IGEW2qgfLpR7h88RAWf/MmxHp6J/lqc+LuYLoOni5FckGNIgOLfgQsAYynp03Lcm
         cnIfQtSGxMe4+XsHkNupelYvRDvzBWLyIo4HY=
Message-ID: <4A0DE954.4020102@gmail.com>
Date: Sat, 16 May 2009 07:14:44 +0900
From: Tejun Heo <htejun@gmail.com>
User-Agent: Thunderbird 2.0.0.19 (X11/20081227)
MIME-Version: 1.0
To: Pete Zaitcev <zaitcev@redhat.com>
CC: Jens Axboe <jens.axboe@oracle.com>,
       James Bottomley <James.Bottomley@HansenPartnership.com>,
       Boaz Harrosh <bharrosh@panasas.com>,
       Linux Kernel <linux-kernel@vger.kernel.org>,
       linux-scsi <linux-scsi@vger.kernel.org>,
       IDE/ATA development list <linux-ide@vger.kernel.org>,
       Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>,
       Borislav Petkov <petkovbb@googlemail.com>,
       Sergei Shtylyov <sshtylyov@ru.mvista.com>,
       Eric Moore <Eric.Moore@lsi.com>, "Darrick J. Wong" <djwong@us.ibm.com>
Subject: Re: [PATCH block#for-2.6.31 2/3] block: set rq->resid_len to blk_rq_bytes()
 on issue
References: <4A0D86DB.9000203@kernel.org>	<4A0D87D2.7090806@gmail.com> <20090515112901.84d4dd97.zaitcev@redhat.com>
In-Reply-To: <20090515112901.84d4dd97.zaitcev@redhat.com>
X-Enigmail-Version: 0.95.7
Content-Type: text/plain; charset=ISO-8859-1
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

Pete Zaitcev wrote:
> On Sat, 16 May 2009 00:18:42 +0900, Tejun Heo <htejun@gmail.com> wrote:
> 
>> In commit c3a4d78c580de4edc9ef0f7c59812fb02ceb037f, while introducing
>> rq->resid_len, the default value of residue count was changed from
>> full count to zero. []
> 
> So it's not a residue anymore, right? You should've renamed it to
> rq->count or something, then. Now we have this:

It still is.  It just is restoring the original behavior.

>> +++ block/drivers/block/ub.c
>> @@ -781,8 +781,7 @@ static void ub_rw_cmd_done(struct ub_dev
>>  
>>  	if (cmd->error == 0) {
>>  		if (blk_pc_request(rq)) {
>> -			if (cmd->act_len < blk_rq_bytes(rq))
>> -				rq->resid_len = blk_rq_bytes(rq) - cmd->act_len;
>> +			rq->resid_len -= min(cmd->act_len, rq->resid_len);
>>  			scsi_status = 0;
> 
> You are subtracting resid_len from itself. Just how in the world
> can this be correct?
>
> Even it if is, in fact, correct, it's such an eggregious violation
> of good style, that your good programmer's card is going to lose
> a big coupon and have a hole punched in it.

The original code was

 if (cmd->act_len >= rq->data_len)
	rq->data_len = 0;
 else
	rq->data_len -= cmd->act_len

So, I could have written

 if (cmd->act_len >= rq->resid_len)
	rq->resid_len = 0;
 else
	rq->resid_len -= cmd->act_len

Instead I wrote

 rq->resid_len -= min(cmd->act_len, rq->resid_len);

It's just capping the amount to be subtracted so that resid_len
doesn't underflow.  What is so wrong or bad style about that?

> This is not in Linus' tree yet, but I'm going to take a hard look
> at this once it shows up.

It would be great if you do before it hits Linus's tree.

Thanks.

-- 
tejun