On Fri, Aug 25, 2017 at 10:34:56AM +0100, Dr. David Alan Gilbert wrote:
> * Peter Xu (pet...@redhat.com) wrote:
> > On Wed, Aug 23, 2017 at 07:01:35PM +0100, Dr. David Alan Gilbert wrote:
> > > * Peter Xu (pet...@redhat.com) 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 <pet...@redhat.com>
> > > > ---
> > > > 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.
This change would suite better for the postcopy recovery series. Will
add one more patch for it.
>
> > >
> > > 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.
Agreed.
--
Peter Xu
