linux-kernel - cfq-iosched.c: panic in cfq_dispatch

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Date:	Sun, 20 Mar 2011 15:23:23 +0800
From:	"Lina Lu" <lulina_nuaa@...mail.com>
To:	"Vivek Goyal" <vgoyal@...hat.com>
Cc:	"linux kernel mailing list" <linux-kernel@...r.kernel.org>
Subject: cfq-iosched.c: panic in cfq_dispatch_requests

Hi Vivek,

I did some test with blkio weight policy these days. The kernel panic 
once in a while. 

>From the calltrace, It 's in func cfq_dispatch_requests(+0x058D).
And I caught a divide by zero error with system monitor tools.

The panic happens when I first set 100 weight to two IO pids, then 
change the weight to 200, and begin to perform IO. The kernel panic
accurately when the IO begin.

I try to find the divide 0 code in cfq_dispatch_requests. This function
call the cfq_select_queue to select a queue, and cfq_select_queue
call cfq_choose_cfqg, then choose_service_tree(). There is a expression
of division in choose_service_tree.

        slice = group_slice * count /
                max_t(unsigned, cfqg->busy_queues_avg[cfqd->serving_prio],
                      cfq_group_busy_queues_wl(cfqd->serving_prio, cfqd, cfqg));

I don't know whether this code divide 0. Can you take a look of this issue?

Thanks
Lina 

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