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Re: [libvirt] [PATCH] qemu: Fix race between async and query jobs

On 12/14/2011 03:29 AM, Jiri Denemark wrote:
> If an async job run on a domain will stop the domain at the end of the
> job, a concurrently run query job can hang in qemu monitor nothing can
> be done with that domain from this point on. An attempt to start such
> domain results in "Timed out during operation: cannot acquire state
> change lock" error.

I think I've actually hit this once or twice, even as hard as it is to
line up the pre-conditions :)

> However, quite a few things have to happen at the right time... There
> must be an async job running, which stops a domain at the end. This race
> was with dump --crash but other similar jobs, such as (managed)save and
> migration, should be able to trigger this bug as well. While this async
> job is processing its last monitor command, that is a query-migrate to
> which qemu replies with status "completed", a new libvirt API that
> results in a query job must arrive and stay waiting until the
> query-migrate command finishes. Once query-migrate is done but before
> the async job closes qemu monitor while stopping the domain, the other
> thread needs to wake up and call qemuMonitorSend to send its command to
> qemu. Before qemu gets a chance to respond to this command, the async
> job needs to close the monitor. At this point, the query job thread is
> waiting for a condition that no-one will ever signal so it never
> finishes the job.

The explanation's longer than the patch!  But that's the key to
understanding things, and you did a good job.

> ---
>  src/qemu/qemu_monitor.c |   14 ++++++++++++++
>  1 files changed, 14 insertions(+), 0 deletions(-)
> diff --git a/src/qemu/qemu_monitor.c b/src/qemu/qemu_monitor.c
> index 4141fb7..35648ae 100644
> --- a/src/qemu/qemu_monitor.c
> +++ b/src/qemu/qemu_monitor.c
> @@ -750,6 +750,20 @@ void qemuMonitorClose(qemuMonitorPtr mon)
>          VIR_FORCE_CLOSE(mon->fd);
>      }
> +    /* In case another thread is waiting for its monitor command to be
> +     * processed, we need to wake it up with appropriate error set.
> +     */
> +    if (mon->msg) {
> +        if (mon->lastError.code == VIR_ERR_OK) {
> +            qemuReportError(VIR_ERR_OPERATION_FAILED,
> +                            _("Qemu monitor was closed"));
> +            virCopyLastError(&mon->lastError);
> +            virResetLastError();
> +        }
> +        mon->msg->finished = 1;
> +        virCondSignal(&mon->notify);
> +    }


Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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