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

[Dr. David Alan Gilbert]

* Peter Xu (address@hidden) wrote:
> 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()?

Yes I think that might work.

> > 
> > 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.

OK; these thigns just emphasise how hard it is to make a function really
lock free.

Dave

> -- 
> Peter Xu
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK