Re: [Qemu-devel] [PATCH v4 3/8] block: add request tracking

[Stefan Hajnoczi]

On Mon, Dec 5, 2011 at 4:09 PM, Marcelo Tosatti <address@hidden> wrote:
> On Wed, Nov 23, 2011 at 11:47:53AM +0000, Stefan Hajnoczi wrote:
> On earlier discussion, you replied to me:
>
> "
>>>      req = tracked_request_add(bs, sector_num, nb_sectors, false);
>>
>> The tracked request should include cluster round info?
>
> When checking A and B for overlap, only one of them needs to be
> rounded in order for overlap detection to be correct.  Therefore we
> can store the original request [start, length) in tracked_requests and
> only round the new request.
> "
>
> The problem AFAICS is this:
>
> - Store a non-cluster-aligned request in the tracked request list.
> - Wait on that non-cluster-aligned request
>  (wait_for_overlapping_requests).
> - Submit cluster-aligned request for COR request.
>
> So, the tracked request list does not properly reflect the in-flight
> COR requests. Which can result in:
>
> 1) guest reads sector 10.
> 2) <sector_num=10,nb_sectors=2> added to tracked request list.
> 3) COR code submits read for <sector_num=10,nb_sectors=2+cluster_align>

It will also round down to sector_num=0 when cluster size is 128
sectors (e.g. qcow2 and qed).

> 4) unrelated guest operation writes to sector 13, nb_sectors=1. That is
> allowed to proceed without waiting because tracked request list does not
> reflect what COR in-flight requests.

The tracked request list has <sector_num=10, nb_sectors=2> and the
candidate write request is <sector_num=13, nb_sectors=1>.

In wait_for_overlapping_requests() we round the candidate request to
<sector_num=0, nb_sectors=cluster_size>.  This rounded request does
overlap <sector_num=10, sectors=2> so it will need to wait.

Therefore CoR and write will not execute at the same time.

Does this make more sense?

Stefan