On Fri, Aug 04, 2017 at 01:46:17PM +0200, Paolo Bonzini wrote:
> On 04/08/2017 11:58, Stefan Hajnoczi wrote:
> >> the root cause of this bug is related to this as well:
> >> https://lists.gnu.org/archive/html/qemu-devel/2017-07/msg02945.html
> >>
> >> From commit 99723548 we started assuming (incorrectly?) that blk_
> >> functions always WILL have an attached BDS, but this is not always true,
> >> for instance, flushing the cache from an empty CDROM.
> >>
> >> Paolo, can we move the flight counter increment outside of the
> >> block-backend layer, is that safe?
> > I think the bdrv_inc_in_flight(blk_bs(blk)) needs to be fixed
> > regardless of the throttling timer issue discussed below.  BB cannot
> > assume that the BDS graph is non-empty.
> 
> Can we make bdrv_aio_* return NULL (even temporarily) if there is no
> attached BDS?  That would make it much easier to fix.

There are many blk_aio_*() callers.  Returning NULL forces them to
perform extra error handling.

When you say "temporarily" do you mean it returns NULL but schedules a
one-shot BH to invoke the callback?  I wonder if we can use a singleton
aiocb instead of NULL for -ENOMEDIUM errors.

Stefan

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