Re: [PATCH] [RFC] make hd_struct->in_flight atomic to avoid diskstat corruption

[Tejun Heo]

Hello,

Jens Axboe wrote:
> On Thu, Apr 16 2009, Tejun Heo wrote:
>> Hello, Nikanth, Jens.
>>
>> Nikanth Karthikesan wrote:
>>>> Hmm. Did you observe this behaviour?
>>> Sorry, not on current kernels. But on a very old 2.6.5 kernel.
>>>
>>> Reading Documentation/iostats.txt and the changelog of commit 
>>> e71bf0d0ee89e51b92776391c5634938236977d5 made me assume that this could be a 
>>> problem even today.
>> The only problem we can run into there is if a request doesn't get
>> attributed to a partition on issue but gets attributed to a partition
>> on completion, which seems to be possible if a new partition is added
>> while IO on the whole device which fell into the new partition area is
>> already in progress, which, on the first glance, seems possible if the
>> admin tries really hard.  I think we can get around the problem by
>> doing part->in_flight = min(max(new_val, part0->in_flight), 0) in
>> dec_in_flight().  This is pretty extreme corner case tho.
> 
> Heh, that is pretty extreme. I'd prefer just quiescing the queue,
> perhaps we should do that for partition map swaps.

Yeah, I think that would be the better approach for swapping ptbl.
RCU isn't really necessary there.

Thanks.

-- 
tejun
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