linux-kernel - Re: [PATCH] cfq-iosched: replace sync_flight by rq_in_driver[BLK_RW

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Message-ID: <20091230184748.GF4489@kernel.dk>
Date:	Wed, 30 Dec 2009 19:47:49 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Corrado Zoccolo <czoccolo@...il.com>
Cc:	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Jeff Moyer <jmoyer@...hat.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Shaohua Li <shaohua.li@...el.com>,
	Gui Jianfeng <guijianfeng@...fujitsu.com>
Subject: Re: [PATCH] cfq-iosched: replace sync_flight by
	rq_in_driver[BLK_RW_SYNC]

On Wed, Dec 30 2009, Corrado Zoccolo wrote:
> According to my intuition (and brief testing), sync_flight is always
> equal to rq_in_driver[BLK_RW_SYNC] at the point of usage, so it can
> be removed and replaced by the other.

They are not fully identical. ->sync_flight is incremented on insertion
on the dispatch list, ->rq_in_driver not until the request is activated
(eg the driver has retrieved it and wants to dispatch to the hardware).
They will usually be identical, but that may not be true for requeues
for instance.

-- 
Jens Axboe

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