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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