Re: [Qemu-devel] [PATCH v6 27/28] migration/qmp: add command migrate-pause

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Xu (peterx@redhat.com) wrote:
> On Tue, Mar 13, 2018 at 12:35:56PM +0000, Dr. David Alan Gilbert wrote:
> 
> [...]
> 
> > > Yes I think if without OOB we should be fine since even the cleanup is
> > > running with the BQL.
> > > 
> > > Now I don't have good idea to solve this problem except introducing a
> > > lock.  How about I add a patch to introduce the mgmt_lock, which
> > > currently only protect the QEMUFile?  Like:
> > > 
> > > ----------------------------------
> > > diff --git a/migration/migration.c b/migration/migration.c
> > > index f31fcbb0d5..00c630326d 100644
> > > --- a/migration/migration.c
> > > +++ b/migration/migration.c
> > > @@ -1195,8 +1195,10 @@ static void migrate_fd_cleanup(void *opaque)
> > >          if (multifd_save_cleanup(&local_err) != 0) {
> > >              error_report_err(local_err);
> > >          }
> > > +        qemu_mutex_lock(&s->mgmt_lock);
> > >          qemu_fclose(s->to_dst_file);
> > >          s->to_dst_file = NULL;
> > > +        qemu_mutex_unlock(&s->mgmt_lock);
> > >      }
> > > 
> > >      assert((s->state != MIGRATION_STATUS_ACTIVE) &&
> > > @@ -2493,8 +2495,10 @@ static MigThrError postcopy_pause(MigrationState *s)
> > > 
> > >          /* Current channel is possibly broken. Release it. */
> > >          assert(s->to_dst_file);
> > > +        qemu_mutex_lock(&s->mgmt_lock);
> > >          qemu_file_shutdown(s->to_dst_file);
> > >          qemu_fclose(s->to_dst_file);
> > >          s->to_dst_file = NULL;
> > > +        qemu_mutex_unlock(&s->mgmt_lock);
> > 
> > That's only safe if we know qemu_fclose() can never block; otherwise
> > we're not allowed to take the same lock in the OOB command.
> > 
> > I think perhaps it's safer to always do something like:
> >   tmp = atomic_xchg(s->to_dst_file, NULL);
> >   qemu_file_shutdown(tmp);
> >   qemu_fclose(tmp);
> > 
> > then the OOB code can do the same?
> > Would that work - avoiding the lock?
> 
> According to what we discussed offlist: I'll still introduce that
> lock, but instead I'll move the close() out of the lock section since
> that can block.

Yep, it feels like that should work.

> I'll see whether I need a repost tomorrow, or after 2.12 release if
> the series will never have a chance for 2.12.

Yes, I think the boat has probably sailed on 2.12; still with the OOB
stuff in, things are much closer.

Dave

> Thanks,
> 
> -- 
> Peter Xu
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK