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

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

* Fam Zheng (famz@redhat.com) wrote:
> On Fri, 09/15 09:42, Dr. David Alan Gilbert wrote:
> > * Fam Zheng (famz@redhat.com) wrote:
> > > 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?
> > 
> > I'm not sure I see an easy fix; I agree with Peter that calling
> > migrate_cancel() followed by qemu_savevm_state_cleanup() is racy,
> > because the cancel just forces the state to CANCELLING before coming
> > back to you, and the migration thread asynchronously starts to
> > fail/cleanup.
> > 
> > migrate_cancel() can forcibly unblock some cases because it calls
> > shutdown(2) on the network fd, but there are other ways for a migration
> > to hang.
> > 
> > Having said that,  the migration thread does it's calls
> > to qemu_savevm_state_cleanup under the lock_iothread;
> > Do we have that lock at this point?
> 
> Yes we do.  Main loop releases the lock only during poll(), other parts of
> main() all have the lock.

Having said that though this is pretty confusing; because at this point
we're after main_loop has exited.
I don't think the migration thread will exit without having taken the
iothread lock; so if we've got it at this point then the migration
thread will never exit, and it will never call qemu_savevm_state_cleanup
itself - so that race might not exist?

However, assuming the migration is at an earlier point, it might be
calling one of the state handlers that you're cleaning up, and that's
racy in individual devices.

If we have the lock I don't think we can wait for the migration to
complete/cancel.   The transition from cancelling->cancelled happens
in migrate_fd_cleanup() - and that's run of a bh which I assume don't
work any more at this point.

Perhaps the answer is to move this to qemu_system_shutdown_request prior
to the point where shutdown_requested is set?  At that point we've
still got the main loop, although hmm I'm not convinced if it's
consistent whether that's called with or without the lock held.

Dave

> Fam
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK