cfq-iosched.c: panic in cfq_dispatch_requests

["Lina Lu" <lulina_nuaa@foxmail.com>]

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