Re: [PATCH v1 1/2] monitor: move monitor destruction to the very end of qemu cleanup

[Markus Armbruster <armbru@redhat.com>]

Denis Plotnikov <den-plotnikov@yandex-team.ru> writes:

> This is needed to keep sending DEVICE_DELETED events on qemu cleanup.
> The event may happen in the rcu thread and we're going to flush the rcu queue
> explicitly before qemu exiting in the next patch. So move the monitor
> destruction to the very end of qemu cleanup to be able to send all the events.
>
> Signed-off-by: Denis Plotnikov <den-plotnikov@yandex-team.ru>
> ---
>  monitor/monitor.c  | 6 ++++++
>  softmmu/runstate.c | 2 +-
>  2 files changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/monitor/monitor.c b/monitor/monitor.c
> index 21c7a68758f5..b04ae4850db2 100644
> --- a/monitor/monitor.c
> +++ b/monitor/monitor.c
> @@ -605,11 +605,17 @@ void monitor_data_init(Monitor *mon, bool is_qmp, bool skip_flush,
>      mon->outbuf = g_string_new(NULL);
>      mon->skip_flush = skip_flush;
>      mon->use_io_thread = use_io_thread;
> +    /*
> +     * take an extra ref to prevent monitor's chardev
> +     * from destroying in qemu_chr_cleanup()
> +     */
> +    object_ref(OBJECT(mon->chr.chr));

I'm not sure we need the comment in the long run.

Taking a reference changes mon->chr.chr from soft reference to hard
reference.  Feels right to me.

Note that mon->chr.chr isn't set here, but earlier.  Unlike the other
parts of @mon.  Because of this it starts as a soft reference, and
hardens only here.

It's set in three places:

1. monitor_init_hmp():

    if (!qemu_chr_fe_init(&mon->common.chr, chr, errp)) {
        g_free(mon);
        return;
    }

    monitor_data_init(&mon->common, false, false, false);

2. monitor_init_qmp():

    if (!qemu_chr_fe_init(&mon->common.chr, chr, errp)) {
        g_free(mon);
        return;
    }
    qemu_chr_fe_set_echo(&mon->common.chr, true);

    /* Note: we run QMP monitor in I/O thread when @chr supports that */
    monitor_data_init(&mon->common, true, false,
                      qemu_chr_has_feature(chr, QEMU_CHAR_FEATURE_GCONTEXT));

3. qmp_human_monitor_command()

    MonitorHMP hmp = {};

    monitor_data_init(&hmp.common, false, true, false);

   hmp.common.chr.chr remains null here.  Works, because
   object_ref(OBJECT(NULL)) and object_unref(OBJECT(NULL)) do nothing.

Slightly cleaner, I think: pass the character device as an argument to
monitor_data_init(), take a reference and store it in mon->chr.chr
there.

>  }
>  
>  void monitor_data_destroy(Monitor *mon)
>  {
>      g_free(mon->mon_cpu_path);
> +    object_unref(OBJECT(mon->chr.chr));
>      qemu_chr_fe_deinit(&mon->chr, false);
>      if (monitor_is_qmp(mon)) {
>          monitor_data_destroy_qmp(container_of(mon, MonitorQMP, common));
> diff --git a/softmmu/runstate.c b/softmmu/runstate.c
> index 10d9b7365aa7..8d29dd2c00e2 100644
> --- a/softmmu/runstate.c
> +++ b/softmmu/runstate.c
> @@ -819,8 +819,8 @@ void qemu_cleanup(void)
>      tpm_cleanup();
>      net_cleanup();
>      audio_cleanup();
> -    monitor_cleanup();
>      qemu_chr_cleanup();
>      user_creatable_cleanup();
> +    monitor_cleanup();
>      /* TODO: unref root container, check all devices are ok */
>  }

Monitor cleanup now runs with character devices that have been
"unparented" from the QOM tree.  Paolo, is that okay?