Re: [Qemu-devel] [PATCH] migration: Cleanup during exit

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

* Peter Xu (peterx@redhat.com) wrote:
> On Wed, Feb 27, 2019 at 04:49:00PM +0000, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > Currently we cleanup the migration object as we exit main after the
> > main_loop finishes; however if there's a migration running things
> > get messy and we can end up with the migration thread still trying
> > to access freed structures.
> > 
> > We now take a ref to the object around the migration thread itself,
> > so the act of dropping the ref during exit doesn't cause us to lose
> > the state until the thread quits.
> > 
> > Cancelling the migration during migration also tries to get the thread
> > to quit.
> > 
> > We do this a bit earlier; so hopefully migration gets out of the way
> > before all the devices etc are freed.
> 
> So does it mean that even with the patch it's still possible the
> migration thread will be accessing device structs that have already
> been freed which can still crash QEMU?

Possibly yes; I'm not sure how to go to the next stage and stop that
case; the consensus seems to be we don't want to explicitly block
during the exit process, so doing a join on the migration thread doesn't
seem to be wanted.

Dave

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