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Date: Tue, 26 Aug 2014 11:57:29 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [RFC PATCH v2 6/8] dma: Implement .cancel_async
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To: Fam Zheng <famz@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>

Il 26/08/2014 11:21, Fam Zheng ha scritto:
> On Tue, 08/26 10:46, Paolo Bonzini wrote:
>> Il 26/08/2014 08:08, Fam Zheng ha scritto:
>>> +    if (dbs->cancelled) {
>>> +        ret = -ECANCELED;
>>> +    }
>>
>> Why is dbs->cancelled necessary?
> 
> Request may complete after bdrv_aio_cancel_async with other status, this flag
> is checked to fix the status to -ECANCELED.

Ah, that's because the operation could be partly incomplete?

I think it would be better to call dma_complete with -ECANCELED, instead
of doing the same dbs->cancelled check twice.  For example, in dma_bdrv_cb:

    if (dbs->sg_cur_index == dbs->sg->nsg || ret < 0) {
        dma_complete(dbs, ret);
        return;
    }
    if (dbs->cancelled) {
        dma_complete(dbs, -ECANCELED);
        return;
    }

>>> +static void dma_aio_cancel_async(BlockDriverAIOCB *acb)
>>> +{
>>> +    DMAAIOCB *dbs = container_of(acb, DMAAIOCB, common);
>>> +
>>> +    trace_dma_aio_cancel(dbs);
>>> +
>>> +    dbs->cancelled = true;
>>> +    if (dbs->acb) {
>>> +        acb = dbs->acb;
>>> +        dbs->acb = NULL;
>>
>> Why do you need to set dbs->acb to NULL, since the callback is going to
>> be called?
> 
> Just copied from dma_aio_cancel, but seems not particularly useful. It reminds
> me that the one in dma_aio_cancel also looks suspicious.

Yeah, that one looks useless too...

Paolo