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Message-ID: <4A573FB9.2090202@kernel.org>
Date: Fri, 10 Jul 2009 22:18:49 +0900
From: Tejun Heo <tj@...nel.org>
To: Christoph Hellwig <hch@...radead.org>
CC: Boaz Harrosh <bharrosh@...asas.com>,
Jens Axboe <jens.axboe@...cle.com>,
Linux Kernel <linux-kernel@...r.kernel.org>,
James Bottomley <James.Bottomley@...senPartnership.com>,
linux-scsi <linux-scsi@...r.kernel.org>,
Niel Lambrechts <niel.lambrechts@...il.com>,
FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH 2/4] block: use the same failfast bits for bio and request
Hello, Christoph.
Christoph Hellwig wrote:
> On Thu, Jul 09, 2009 at 09:45:24AM +0900, Tejun Heo wrote:
>> What's more disturbing to me is the different between RQ and BIO
>> flags. __REQ_* are bit positions, REQ_* are masks while BIO_* are bit
>> positions. Sadly it seems it's already too late to change that. I
>> personally an not a big fan of simple accessors or flags defined as
>> bit positions. They seem to obscure things without much benefit.
>
> flags as bit positions generally only make sense if you use
> test/set/clear_bit, otherwise they just confuse things.
Another shortcoming of bit position flags is masking / multi flag
operations. It's just awful. I think it's always better to define
flags as masks even when it's used with test/set/clear_bit(). If such
usages are common enough, we can easily add test/set/clear_bit_mask().
The conversion from mask to bit would be constant most of the time and
it's not like fls/ffs() are expensive.
> And the accessors are pretty annoying, especially in the block
> layer. Trying to find the places where a BIO flag has an actual
> effect is pretty painful due to the mix of the different flags and
> the accessors.
Yeap, fully agreed.
Thanks.
--
tejun
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