linux-kernel - Re: [PATCH] cfq-iosched: Fix a gcc 4.5 warning and put some comments

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Message-ID: <x497hhbe117.fsf@segfault.boston.devel.redhat.com>
Date:	Thu, 21 Oct 2010 16:36:20 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Jens Axboe <jaxboe@...ionio.com>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] cfq-iosched: Fix a gcc 4.5 warning and put some comments

Vivek Goyal <vgoyal@...hat.com> writes:

> - Andi encountedred following warning with gcc 4.5
>
>   linux/block/cfq-iosched.c: In function ‘cfq_dispatch_requests’:
>   linux/block/cfq-iosched.c:2156:3: warning: array subscript is above array
>   bounds
>
> - Warning happens due to following code.
>
>   slice = group_slice * count /
> 		max_t(unsigned, cfqg->busy_queues_avg[cfqd->serving_prio],
> 		cfq_group_busy_queues_wl(cfqd->serving_prio, cfqd, cfqg));
>
>   gcc is complaining about cfqg->busy_queues_avg[] being indexed by CFQ
>   prio classes (RT, BE and IDLE) while the array size is only 2.
>
> - At run time, we never access cfqg->busy_queues_avg[IDLE] and return from
>   function before this code hits.
>
> - To fix warning increase the array size though it will remain unused. This
>   patch also puts some comments to clarify some of the confusions.
>
> - I have taken Jens's patch and modified it a bit.
>
> - Compile tested with gcc 4.4 and boot tested. I don't have gcc 4.5
>   running, Andi can you please test it with gcc 4.5 to make sure it
>   worked.
>
> Reported-by: Andi Kleen <ak@...ux.intel.com>
> Signed-off-by: Vivek Goyal <vgoyal@...hat.com>
> Signed-off-by: Jens Axboe <axboe@...nel.dk>

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