Re: [Qemu-devel] [PATCH 2/3] migration: Cancel migration at exit

[Fam Zheng <famz@redhat.com>]

On Fri, 09/15 16:03, Peter Xu wrote:
> On Fri, Sep 15, 2017 at 01:44:03PM +0800, Fam Zheng wrote:
> > bdrv_close_all() would abort() due to op blockers added by BMDS, clean
> > up migration states when main loop quits to avoid that.
> > 
> > Signed-off-by: Fam Zheng <famz@redhat.com>
> > ---
> >  include/migration/misc.h | 1 +
> >  migration/migration.c    | 7 ++++++-
> >  vl.c                     | 3 +++
> >  3 files changed, 10 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/migration/misc.h b/include/migration/misc.h
> > index c079b7771b..b9a26b0898 100644
> > --- a/include/migration/misc.h
> > +++ b/include/migration/misc.h
> > @@ -54,5 +54,6 @@ bool migration_has_failed(MigrationState *);
> >  /* ...and after the device transmission */
> >  bool migration_in_postcopy_after_devices(MigrationState *);
> >  void migration_global_dump(Monitor *mon);
> > +void migrate_cancel(void);
> >  
> >  #endif
> > diff --git a/migration/migration.c b/migration/migration.c
> > index 959e8ec88e..2c844945c7 100644
> > --- a/migration/migration.c
> > +++ b/migration/migration.c
> > @@ -1274,11 +1274,16 @@ void qmp_migrate(const char *uri, bool has_blk, bool blk,
> >      }
> >  }
> >  
> > -void qmp_migrate_cancel(Error **errp)
> > +void migrate_cancel(void)
> >  {
> >      migrate_fd_cancel(migrate_get_current());
> >  }
> >  
> > +void qmp_migrate_cancel(Error **errp)
> > +{
> > +    migrate_cancel();
> > +}
> > +
> 
> Nit: I would prefer just call migrate_fd_cancel() below, since I don't
> see much point to introduce migrate_cancel() which only calls
> migrate_fd_cancel()...

migrate_get_current() is a migration internal IMHO. But that can be moved to
migrate_fd_cancel() so the parameter is dropped.

> 
> >  void qmp_migrate_set_cache_size(int64_t value, Error **errp)
> >  {
> >      MigrationState *s = migrate_get_current();
> > diff --git a/vl.c b/vl.c
> > index fb1f05b937..abbe61f40b 100644
> > --- a/vl.c
> > +++ b/vl.c
> > @@ -87,6 +87,7 @@ int main(int argc, char **argv)
> >  #include "sysemu/blockdev.h"
> >  #include "hw/block/block.h"
> >  #include "migration/misc.h"
> > +#include "migration/savevm.h"
> >  #include "migration/snapshot.h"
> >  #include "migration/global_state.h"
> >  #include "sysemu/tpm.h"
> > @@ -4799,6 +4800,8 @@ int main(int argc, char **argv, char **envp)
> >      iothread_stop_all();
> >  
> >      pause_all_vcpus();
> > +    migrate_cancel();
> 
> IIUC this is an async cancel, so when reach here the migration thread
> can still be alive.  Then...
> 
> > +    qemu_savevm_state_cleanup();
> 
> ... Here calling qemu_savevm_state_cleanup() may be problematic if
> migration thread has not yet quitted.
> 
> I'm thinking whether we should make migrate_fd_cancel() wait until the
> migration thread finishes (state change to CANCELLED).  Then the
> migration thread will do the cleanup, and here we can avoid calling
> qemu_savevm_state_cleanup() as well.

But if the migration thread is stuck and CANCELLED is never reached, we'll hang
here?

Fam