Re: [PATCHSET] mempool, percpu, blkcg: fix percpu stat allocationand remove stats_lock

[Tejun Heo]

Hello,

On Mon, Feb 27, 2012 at 09:40:45AM -0500, Vivek Goyal wrote:
> So we call following.
> 
> elevator_switch()
>   blkg_destroy_all()
>     update_root_blkg();
> 
> Here update_root_blkg() will free up the blkg->pd for cfq. 
> 
> And later we call.
> 
> elevator_exit()
>   cfq_exit_queue()
>     cfq_put_async_queues()
>       cfq_put_queue()
>         blkg_put(cfqg_to_blkg(cfqg)); <------ trying to reach blkg through
> already freed policy data.
> 
> So we should not free up root group policy data till old elevator has
> exited.

Yeah, this analysis seems correct.  For some reason, I still can't
reproduce it even with slab debugging turned on.  I modified the free
path to fill *pd with pattern w/o actually freeing it and now can see
similar crashes.  Will post fix soon.

Thanks a lot.

-- 
tejun
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