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

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Date:	Wed, 15 Jul 2009 18:27:57 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Boaz Harrosh <bharrosh@...asas.com>
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

Hello, Boaz.

Boaz Harrosh wrote:
>>> 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.

Block flags look like the way they do today because a while back they
were accessed with atomic bitops (the versions w/o the underbars).
Now that they're all inside spinlocks, it all became moot.

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

req->cmd_flags |= QUEUE_FLAG_QUEUED / &= ~QUEUE_FLAG_QUEUED might not
be as clear but should be sufficient, I suppose.

> 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);
...
> 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.

Heh.. I don't know.  What about things like flags & mask == mask2
test?  The vararg macros would work for most cases and I wouldn't be
violently against them if they were already in place but I don't see
much benefit of all those when people are already very accustomed to
using c bitops to handle flags.

Thanks.

-- 
tejun
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