Re: [Qemu-devel] Assertion failure taking external snapshot with virtio drive + iothread

[Ed Swierk]

On Mon, Mar 20, 2017 at 10:26 PM, Fam Zheng <address@hidden> wrote:
> On Fri, 03/17 09:55, 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.
>
>
> Can you test this one?
>
> ---
>
>
> diff --git a/blockdev.c b/blockdev.c
> index c5b2c2c..4c217d5 100644
> --- a/blockdev.c
> +++ b/blockdev.c
> @@ -1772,6 +1772,8 @@ static void external_snapshot_prepare(BlkActionState 
> *common,
>          return;
>      }
>
> +    bdrv_set_aio_context(state->new_bs, state->aio_context);
> +
>      /* This removes our old bs and adds the new bs. This is an operation that
>       * can fail, so we need to do it in .prepare; undoing it for abort is
>       * always possible. */
> @@ -1789,8 +1791,6 @@ static void external_snapshot_commit(BlkActionState 
> *common)
>      ExternalSnapshotState *state =
>                               DO_UPCAST(ExternalSnapshotState, common, 
> common);
>
> -    bdrv_set_aio_context(state->new_bs, state->aio_context);
> -
>      /* We don't need (or want) to use the transactional
>       * bdrv_reopen_multiple() across all the entries at once, because we
>       * don't want to abort all of them if one of them fails the reopen */

With this change, a different assertion fails on running snapshot_blkdev:

  qemu-system-x86_64: /x/qemu/block/io.c:164: bdrv_drain_recurse:
Assertion `qemu_get_current_aio_context() == qemu_get_aio_context()'
failed.

--Ed