Re: [Qemu-devel] [RFC v2 8/8] migration: add incoming mgmt lock

[Peter Xu]

On Wed, Aug 23, 2017 at 07:01:35PM +0100, Dr. David Alan Gilbert wrote:
> * Peter Xu (address@hidden) wrote:
> > Now at least migrate_incoming can be run in parallel.  Let's provide a
> > migration lock to protect it.
> > 
> > Signed-off-by: Peter Xu <address@hidden>
> > ---
> >  migration/migration.c | 6 ++++++
> >  migration/migration.h | 3 +++
> >  2 files changed, 9 insertions(+)
> > 
> > diff --git a/migration/migration.c b/migration/migration.c
> > index c3fe0ed..32058f7 100644
> > --- a/migration/migration.c
> > +++ b/migration/migration.c
> > @@ -145,6 +145,7 @@ MigrationIncomingState 
> > *migration_incoming_get_current(void)
> >          mis_current.state = MIGRATION_STATUS_NONE;
> >          memset(&mis_current, 0, sizeof(MigrationIncomingState));
> >          qemu_mutex_init(&mis_current.rp_mutex);
> > +        qemu_mutex_init(&mis_current.mgmt_mutex);
> >          qemu_event_init(&mis_current.main_thread_load_event, false);
> >          once = true;
> >      }
> > @@ -1171,6 +1172,7 @@ void qmp_migrate_incoming(const char *uri, Error 
> > **errp)
> >  {
> >      Error *local_err = NULL;
> >      static bool once = true;
> > +    MigrationIncomingState *mis = migration_incoming_get_current();
> 
> migration_incoming_get_current isn't actually thread-safe itself unless
> you can guarantee the initial allocation has happened - otherwise both
> threads can race and do the 'once' code at the same time.

How about I init the incoming object as well in
migration_object_init()?

> 
> Similarly, these locks - they don't protect our 'once' - so a second
> thread could come in here and both get past the !once check.

Oh I missed this one since actually I am removing that "once" variable
in postcopy recovery series. :)

I can put the last two patches into postcopy recovery series, then
it'll be fine.

-- 
Peter Xu