Re: [Qemu-devel] qemu 2.0, deadlock in block-commit

[Stefan Hajnoczi]

On Mon, May 26, 2014 at 02:58:35PM +0200, Marcin Gibuła wrote:
> >Two options for making progress on this bug:
> >
> >1. Debug bdrv_drain_all() and find out whether there are any I/O
> >    requests remaining.
> 
> Yes, there is one request pending on active layer of disk that is being
> commited (on bs->tracked_requests list). IO threads die off because they
> have nothing to do... it seems that requests are somehow not commited into
> threads.
> 
> I tried hard (and will continue to try) to debug this, but documentation is
> limited :-) so ANY tips where to look are welcome.
> 
> >2. Post steps for reproducing this problem (exact command-lines or virsh
> >    commands used).
> 
> I'm using application that talks with libvirt via API, so I describe what it
> does.
> 
> 1. Create a VM, boot a system. I'm using iso from http://www.sysresccd.org
> 
> 2. VM has a mounted QCOW2 disk with following hierarchy:
> [file1] -> [file2 (active)]
> 
> Both are qcow2 files.
> 
> 3. Open console. Start command:
> 
> while true; do dd if=/dev/zero of=/dev/vdX bs=512k oflag=direct; done;
> 
> Where vdX is of course qcow2 disk described above.
> 
> 4. Create snapshot of file2 (virDomainSnapshotCreateXML). So now we have:
> 
> [file1] -> [file2] -> [file3 (active)] -
> 
> 5. Wait couple of seconds (so snapshot fills up).
> 
> 6. Commit file2 into file1 (virDomainBlockCommit).
> 
> 7. During commit, another threads is using virDomainGetBlockJobInfo() to
> query its progress.
> 
> Note - it doesn't always happen. I have about 1 per 10 failure rate with
> this procedure.
> 
> Do you want me to reproduce it manually with pure virsh?

What happens if you omit #7 virDomainGetBlockJobInfo()?  Does it still
hang 1/10 times?

Can you post the QEMU command-line so we know the precise VM
configuration?  (ps aux | grep qemu)

Thanks,
Stefan