linux-kernel - Re: [PATCH block#for-2.6.31 2/3] block: set rq->resid_len to blk_rq

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Date:	Sat, 16 May 2009 07:14:44 +0900
From:	Tejun Heo <htejun@...il.com>
To:	Pete Zaitcev <zaitcev@...hat.com>
CC:	Jens Axboe <jens.axboe@...cle.com>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Boaz Harrosh <bharrosh@...asas.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	IDE/ATA development list <linux-ide@...r.kernel.org>,
	Bartlomiej Zolnierkiewicz <bzolnier@...il.com>,
	Borislav Petkov <petkovbb@...glemail.com>,
	Sergei Shtylyov <sshtylyov@...mvista.com>,
	Eric Moore <Eric.Moore@....com>,
	"Darrick J. Wong" <djwong@...ibm.com>
Subject: Re: [PATCH block#for-2.6.31 2/3] block: set rq->resid_len to blk_rq_bytes()
 on issue

Pete Zaitcev wrote:
> On Sat, 16 May 2009 00:18:42 +0900, Tejun Heo <htejun@...il.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
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