linux-kernel - Re: [PATCH 2/4] block: use the same failfast bits for bio and request

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Date:	Sun, 12 Jul 2009 15:06:03 +0300
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Christoph Hellwig <hch@...radead.org>,
	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

On 07/10/2009 04:18 PM, Tejun Heo wrote:
> 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.
> 

first please make a distinction between test/set/clear_bit and
test/__set/__clear_bit the former is not an option since it's not what
we need.

I too, do not like the lower-case accessors for upper-case bits like:
blk_failfast_dev() && blk_failfast_transport() which give nothing
and confuse the grepping of sets-vs-clears.

But I do like the use of __set/__clear_bit of flags. grepping is clear
and code semantics are more correct. Actually I prefer when a construct
like bio or request have two accessors set/clear_flags, which abstract
out not the bits but the flags member. Say when things evolve in the future
it is easer to adapted.

What can be more clear then rq_set_flags(req, QUEUE_FLAG_QUEUED) then
rq_clear_flags(req, QUEUE_FLAG_QUEUED) later.

> 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.
> 

That's why I suggested the set/clear_flags() variable size macro
which can  set/clear multiple bit-flags at same cost of masks, only
that the compiler calculates the mask in compile time.

This can also be good for the greps above. .eg:
test_flags(&rq->cmd_flags, REQ_FAILFAST_DEV, REQ_FAILFAST_TRANSPORT, REQ_FAILFAST_DRIVER);

>> 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.
> 

As said, yes, the the lower-case accessors for upper-case bits does nothing,
but use __set/__clear/test is a different matter that can also replace the
sugary need of these.

> Thanks.
> 

Thanks
Boaz
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