* Juan Quintela (quint...@redhat.com) wrote:
> "Dr. David Alan Gilbert" <dgilb...@redhat.com> wrote:
> > Hi Alex,
> > Can you see if the attached patch fixes the ahci/migrate failure you
> > see; it won't fail for me however mean I am to it.
> >
> >
>
> ....
>
> > void migration_object_finalize(void)
> > {
> > + /*
> > + * Cancel the current migration - that will (eventually)
> > + * stop the migration using this structure
> > + */
> > + migrate_fd_cancel(current_migration);
>
> This can only happen during "civilized" exit of qemu, right?
> Otherwise, we are changing the migration status.
I'm hoping that, in an exit where:
a) there was no migration this does nothing
b) the migration finished already this does nothing
c) There's a running migration then this causes a cancel.
So if (a) & (b) do nothing then this should be OK, and
if you're trying to do an exit during (c) then it's going bad anyway.
> > object_unref(OBJECT(current_migration));
> > }
> >
> > @@ -3134,6 +3140,7 @@ static void *migration_thread(void *opaque)
> >
> > rcu_register_thread();
> >
> > + object_ref(OBJECT(s));
>
> It is weird that this is not enough :-(
It maybe enough; I was being cautious to add the cancel as well -
I'm pretty sure this series doesn't cover every case, but since these
bugs are hard to reproduce I wanted to try and cover any and all easy
cases. There was a previous attempt (by Fam) 2 years ago which stalled
because we couldn't figure out how to cover everything.
> > s->iteration_start_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
> >
> > qemu_savevm_state_header(s->to_dst_file);
> > @@ -3230,6 +3237,7 @@ static void *migration_thread(void *opaque)
> >
> > trace_migration_thread_after_loop();
> > migration_iteration_finish(s);
> > + object_unref(OBJECT(s));
> > rcu_unregister_thread();
> > return NULL;
> > }
> > diff --git a/vl.c b/vl.c
> > index 2f340686a7..c1920165f3 100644
> > --- a/vl.c
> > +++ b/vl.c
> > @@ -4579,6 +4579,12 @@ int main(int argc, char **argv, char **envp)
> >
> > gdbserver_cleanup();
> >
> > + /*
> > + * cleaning up the migration object cancels any existing migration
> > + * try to do this early so that it also stops using devices.
> > + */
> > + migration_object_finalize();
> > +
> > /* No more vcpu or device emulation activity beyond this point */
> > vm_shutdown();
> >
> > @@ -4594,7 +4600,6 @@ int main(int argc, char **argv, char **envp)
> > monitor_cleanup();
> > qemu_chr_cleanup();
> > user_creatable_cleanup();
> > - migration_object_finalize();
> > /* TODO: unref root container, check all devices are ok */
> >
> > return 0;
>
> Ok, it was happening really late.
>
> Once that you are at this, can we rename it?
>
> migration_cleanup()
>
> looks more consistent with everything else, and makes sure that it does
> "more" than finalize the object, no?
Sure, I'll go with migration_shutdown
> Furthermore, it is enough to "just" cancel the migration? It is not
> needed that we wait for the migration thread to finish?
I agree, but the problem is I don't want to block in the exit path - so
what are the choices?
> If the problem was that migration_thread() was accessing the object
> after main thread freed it, then just the ref counting should be enough,
> no? My understanding is that returning from main is the quivalent of
> exit() and kill all threads? Or are we doing something special for that
> not to happen?
The case Alex saw was gprof having an atexit handler so main hadn't
really exited yet and hadn't killed it's threads; having said that I
can't trigger this even with gprof.
Dave
> Later, Juan.
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
