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

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Date:	Fri, 15 May 2009 17:16:45 -0600
From:	Pete Zaitcev <zaitcev@...hat.com>
To:	Tejun Heo <htejun@...il.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>, zaitcev@...hat.com
Subject: Re: [PATCH block#for-2.6.31 2/3] block: set rq->resid_len to
 blk_rq_bytes() on issue

On Sat, 16 May 2009 07:14:44 +0900, Tejun Heo <htejun@...il.com> wrote:

> 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?

Curse of the gifted, I guess. To use a subtraction instead of zero
this way looks like a pointless, even mischievous obfuscation to me.

Also, we probably want a stack_dump or a printk when actual length
exceeds the requested length, don't we? If it ever happens, we
might be overwriting some I/O buffer somewhere.

-- Pete
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