Re: [Qemu-devel] [PATCH v2 5/6] monitor: prevent inserting new monitors after cleanup

[Markus Armbruster <armbru@redhat.com>]

Marc-André Lureau <marcandre.lureau@redhat.com> writes:

> Add a monitor_destroyed global to check if monitor_cleanup() has been
> already called. In this case, don't insert the new monitor in the
> list, but free it instead.
>
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

The commit message explains what the patch does, but not why we want to
do it.

> ---
>  monitor.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/monitor.c b/monitor.c
> index fffeb27ef9..7fe89daa87 100644
> --- a/monitor.c
> +++ b/monitor.c
> @@ -263,10 +263,11 @@ typedef struct QMPRequest QMPRequest;
>  /* QMP checker flags */
>  #define QMP_ACCEPT_UNKNOWNS 1
>  
> -/* Protects mon_list, monitor_qapi_event_state.  */
> +/* Protects mon_list, monitor_qapi_event_state, monitor_destroyed.  */
>  static QemuMutex monitor_lock;
>  static GHashTable *monitor_qapi_event_state;
>  static QTAILQ_HEAD(mon_list, Monitor) mon_list;
> +static bool monitor_destroyed;
>  
>  /* Protects mon_fdsets */
>  static QemuMutex mon_fdsets_lock;
> @@ -4536,8 +4537,16 @@ void error_vprintf_unless_qmp(const char *fmt, va_list ap)
>  static void monitor_list_append(Monitor *mon)
>  {
>      qemu_mutex_lock(&monitor_lock);
> -    QTAILQ_INSERT_HEAD(&mon_list, mon, entry);
> +    if (!monitor_destroyed) {
> +        QTAILQ_INSERT_HEAD(&mon_list, mon, entry);
> +        mon = NULL;
> +    }
>      qemu_mutex_unlock(&monitor_lock);
> +
> +    if (mon) {
> +        monitor_data_destroy(mon);
> +        g_free(mon);
> +    }
>  }
>  
>  static void monitor_qmp_setup_handlers_bh(void *opaque)
> @@ -4631,6 +4640,7 @@ void monitor_cleanup(void)
>  
>      /* Flush output buffers and destroy monitors */
>      qemu_mutex_lock(&monitor_lock);
> +    monitor_destroyed = true;
>      QTAILQ_FOREACH_SAFE(mon, &mon_list, entry, next) {
>          QTAILQ_REMOVE(&mon_list, mon, entry);
>          monitor_flush(mon);

monitor_cleanup() is one of the last things main() calls before it
returns.  If another thread creates a monitor afterwards, it won't be
cleaned up.  I figure that's the reason we want this patch.  There might
be more serious issues than failure to clean up.  Please explain in your
commit message.

monitor_list_append() is called by monitor_init(), either directly right
before it returns, or via a bottom half if mon->use_io_thread.
Therefore, @monitor_destroyed can only be true if monitor_init()
commonly runs in a thread other than the main thread.  Please give an
example where it does, in your commit message.