Re: [Qemu-devel] [PATCH 2/2] monitor: add proper reference counting of the current CPU

[Greg Kurz <groug@kaod.org>]

On Fri, 13 Oct 2017 11:24:59 +0200
Igor Mammedov <imammedo@redhat.com> wrote:

> On Fri, 13 Oct 2017 10:35:31 +0200
> Greg Kurz <groug@kaod.org> wrote:
> 
> > If a CPU selected with the "cpu" command is hot-unplugged then "info cpus"
> > causes QEMU to exit:
> > 
> > (qemu) device_del cpu1
> > (qemu) info cpus
> > qemu:qemu_cpu_kick_thread: No such process
> > 
> > This happens because "cpu" stores the pointer to the selected CPU into
> > the monitor structure. When the CPU is hot-unplugged, we end up with a
> > dangling pointer. The "info cpus" command then does:
> > 
> > hmp_info_cpus()
> >  monitor_get_cpu_index()
> >   mon_get_cpu()
> >    cpu_synchronize_state() <--- called with dangling pointer
> > 
> > This could cause a QEMU crash as well.
> > 
> > This patch switches the monitor to use object_ref() to ensure the
> > CPU object doesn't vanish unexpectedly. The reference is dropped
> > either when "cpu" is used to switch to another CPU, or when the
> > selected CPU is unrealized and cpu_list_remove() sets its cpu_index
> > back to UNASSIGNED_CPU_INDEX.  
> I don't really like an idea of leaving dangling cpu around.
> Is it possible to store QOM path on set_cpu in monitor and
> resolving it each to instance each time it's needed?
> 

It sounds workable. Also it would allow the fix to not depend on patch 1
for ppc.

Thanks for the suggestion!

> 
> > Signed-off-by: Greg Kurz <groug@kaod.org>
> > ---
> >  monitor.c |   12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> > 
> > diff --git a/monitor.c b/monitor.c
> > index fe0d1bdbb461..1c0b9a2c3ad3 100644
> > --- a/monitor.c
> > +++ b/monitor.c
> > @@ -579,6 +579,9 @@ static void monitor_data_init(Monitor *mon)
> >  
> >  static void monitor_data_destroy(Monitor *mon)
> >  {
> > +    if (mon->mon_cpu) {
> > +        object_unref((Object *) mon->mon_cpu);
> > +    }
> >      qemu_chr_fe_deinit(&mon->chr, false);
> >      if (monitor_is_qmp(mon)) {
> >          json_message_parser_destroy(&mon->qmp.parser);
> > @@ -1047,12 +1050,21 @@ int monitor_set_cpu(int cpu_index)
> >      if (cpu == NULL) {
> >          return -1;
> >      }
> > +    if (cur_mon->mon_cpu) {
> > +        object_unref((Object *) cur_mon->mon_cpu);
> > +    }
> >      cur_mon->mon_cpu = cpu;
> > +    object_ref((Object *) cpu);
> >      return 0;
> >  }
> >  
> >  CPUState *mon_get_cpu(void)
> >  {
> > +    if (cur_mon->mon_cpu &&
> > +        cur_mon->mon_cpu->cpu_index == UNASSIGNED_CPU_INDEX) {
> > +        object_unref((Object *) cur_mon->mon_cpu);
> > +        cur_mon->mon_cpu = NULL;
> > +    }
> >      if (!cur_mon->mon_cpu) {
> >          if (!first_cpu) {
> >              return NULL;
> > 
> >   
>