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Date: Mon, 24 Jun 2013 15:49:21 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 1/2] iscsi: add support for
	bdrv_co_is_allocated()
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To: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Peter Lieven <pl@kamp.de>, qemu-devel@nongnu.org, ronniesahlberg@gmail.com

Il 24/06/2013 10:13, Stefan Hajnoczi ha scritto:
> On Fri, Jun 21, 2013 at 11:18:42AM +0200, Kevin Wolf wrote:
>> Am 20.06.2013 um 20:20 hat Peter Lieven geschrieben:
>>> @@ -800,6 +801,60 @@ iscsi_getlength(BlockDriverState *bs)
>>>      return len;
>>>  }
>>>  
>>> +static int coroutine_fn iscsi_co_is_allocated(BlockDriverState *bs,
>>> +                                              int64_t sector_num,
>>> +                                              int nb_sectors, int *pnum)
>>> +{
>>> +    IscsiLun *iscsilun = bs->opaque;
>>> +    struct scsi_task *task = NULL;
>>> +    struct scsi_get_lba_status *lbas = NULL;
>>> +    struct scsi_lba_status_descriptor *lbasd = NULL;
>>> +    int ret;
>>> +
>>> +    *pnum = nb_sectors;
>>> +
>>> +    if (iscsilun->lbpme == 0) {
>>> +        return 1;
>>> +    }
>>> +
>>> +    /* in-flight requests could invalidate the lba status result */
>>> +    while (iscsi_process_flush(iscsilun)) {
>>> +        qemu_aio_wait();
>>> +    }
>>
>> Note that you're blocking here. The preferred way would be something
>> involving a yield from the coroutine and a reenter as soon as all
>> requests are done. Maybe a CoRwLock does what you need?
> 
> The other option is to avoid synchronization here and instead process
> bs->tracked_requests so that any in-flight writes count as allocated.

I think it's a bug if the caller doesn't take into account in-flight
requests.  For example mirroring expects writes to mark sectors as
dirty, which will pick up everything that is_allocated fails to pick up.

If all else fails, you can always add a bdrv_drain_all before the query.

Hence, this check is not needed.  In fact, raw-posix does not perform it.

Paolo