Re: [PATCH v9 5/8] migration/ram.c: add a notifier chain for precopy

[Wei Wang <wei.w.wang@intel.com>]

On 11/29/2018 01:10 PM, Peter Xu wrote:
> On Thu, Nov 29, 2018 at 11:40:57AM +0800, Wei Wang wrote:
> I think this precopy notifier callchain is expected to be used only for
> the precopy mode. Postcopy has its dedicated notifier callchain that
> users could use.
>
> How about changing the migrate_postcopy() check to "ms->start_postcopy":
>
> bool migration_postcopy_start(void)
> {
>      MigrationState *s;
>
>      s = migrate_get_current();
>
>      return atomic_read(&s->start_postcopy);
> }
>
>
> static void precopy_notify(PrecopyNotifyReason reason)
> {
>      if (migration_postcopy_start())
>          return;
>
>      notifier_list_notify(&precopy_notifier_list, &reason);
> }
>
> If postcopy started with precopy, the precopy optimization feature
> could still be used until it switches to the postcopy mode.
> I'm not sure we can use start_postcopy.  It's a variable being set in
> the QMP handler but it does not mean postcopy has started.  I'm afraid
> there can be race where it's still precopy but the variable is set so
> event could be missed...

Peter,
I just found that migration_bitmap_sync is also called by 
ram_postcopy_send_discard_bitmap().
But we don't expect notifier_list_notify to be called in the postcopy mode.

So, probably we still need the start_postcopy check in 
notifier_list_notify though we have
the COMPLETE notifier.

Best,
Wei