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


From: Paolo Bonzini
Subject: Re: [Qemu-devel] Assertion failure taking external snapshot with virtio drive + iothread
Date: Fri, 17 Mar 2017 19:10:37 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0


On 17/03/2017 18:32, Ed Swierk wrote:
> On Fri, Mar 17, 2017 at 10:15 AM, Paolo Bonzini <address@hidden> 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 in particular to this:

+    bdrv_set_backing_hd(bs_new, bs_top, &local_err);
+    if (local_err) {
+        error_propagate(errp, local_err);
+        goto out;
+    }

     change_parent_backing_link(bs_top, bs_new);
-    /* FIXME Error handling */
-    bdrv_set_backing_hd(bs_new, bs_top, &error_abort);


Paolo



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