linux-kernel - Re: [PATCH] cfq-iosched: don't check cfqg in choose_service

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Message-ID: <x49mxo88f9e.fsf@segfault.boston.devel.redhat.com>
Date:	Tue, 14 Dec 2010 11:56:29 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Gui Jianfeng <guijianfeng@...fujitsu.com>
Cc:	Jens Axboe <axboe@...nel.dk>, Vivek Goyal <vgoyal@...hat.com>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cfq-iosched: don't check cfqg in choose_service_tree()

Gui Jianfeng <guijianfeng@...fujitsu.com> writes:

> When cfq_choose_cfqg() is called in select_queue(), there must be at least one
> backlogged CFQ queue waiting for dispatching, hence there must be at least one
> backlogged CFQ group on service tree. So we never call choose_service_tree()
> with cfqg == NULL.

Further, if cfqg was NULL, cfq_choose_cfqg would have dereferenced a
NULL pointer and we wouldn't even get to choose_service_tree.

Reviewed-by: Jeff Moyer <jmoyer@...hat.com>
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