On 17/03/2017 18:32, Ed Swierk wrote:
> On Fri, Mar 17, 2017 at 10:15 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>>
>>
>> On 17/03/2017 18:11, Paolo Bonzini wrote:
>>>
>>>
>>> On 17/03/2017 17:55, Ed Swierk wrote:
>>>> I'm running into the same problem taking an external snapshot with a
>>>> virtio-blk drive with iothread, so it's not specific to virtio-scsi.
>>>> Run a Linux guest on qemu master
>>>>
>>>>   qemu-system-x86_64 -nographic -enable-kvm -monitor
>>>> telnet:0.0.0.0:1234,server,nowait -m 1024 -object
>>>> iothread,id=iothread1 -drive file=/x/drive.qcow2,if=none,id=drive0
>>>> -device virtio-blk-pci,iothread=iothread1,drive=drive0
>>>>
>>>> Then in the monitor
>>>>
>>>>   snapshot_blkdev drive0 /x/snap1.qcow2
>>>>
>>>> qemu bombs with
>>>>
>>>>   qemu-system-x86_64: /x/qemu/include/block/aio.h:457:
>>>> aio_enable_external: Assertion `ctx->external_disable_cnt > 0' failed.
>>>>
>>>> whereas without the iothread the assertion failure does not occur.
>>>
>>> Please try this patch:
>>
>> Hmm, no.  I'll post the full fix on top of John Snow's patches.
> 
> OK. Incidentally, testing with virtio-blk I bisected the assertion
> failure to b2c2832c6140cfe3ddc0de2d77eeb0b77dea8fd3 ("block: Add Error
> parameter to bdrv_append()").

And this is a fix, but I have no idea why/how it works and what else it 
may break.

Patches 1 and 2 are pretty obvious and would be the first step towards 
eliminating aio_disable/enable_external altogether.

However I got patch 3 more or less by trial and error, and when I 
thought I had the reasoning right I noticed this:

        bdrv_drained_end(state->old_bs);

in external_snapshot_clean which makes no sense given the

        bdrv_drained_begin(bs_new);

that I added to bdrv_append.  So take this with a ton of salt.

The basic idea is that calling child->role->drained_begin and 
child->role->drained_end is not necessary and in fact actively wrong 
when both the old and the new child should be in a drained section.  
But maybe instead it should be asserted that they are, except for the 
special case of adding or removing a child.  i.e. after

    int drain = !!(old_bs && old_bs->quiesce_counter) - !!(new_bs && new_bs->quiesce_counter);

add

    assert(!(drain && old_bs && new_bs));

Throwing this out because it's Friday evening... Maybe Fam can pick
it up on Monday.

Paolo