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Re: [PATCH 2/5] migration: Shutdown src in await_return_path_close_on_so
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [PATCH 2/5] migration: Shutdown src in await_return_path_close_on_source() |
Date: |
Wed, 21 Jul 2021 19:00:24 +0100 |
User-agent: |
Mutt/2.0.7 (2021-05-04) |
* Peter Xu (peterx@redhat.com) wrote:
> On Wed, Jul 21, 2021 at 10:55:00AM +0100, Dr. David Alan Gilbert wrote:
> > * Peter Xu (peterx@redhat.com) wrote:
> > > We have a logic in await_return_path_close_on_source() that we will
> > > explicitly
> > > shutdown the socket when migration encounters errors. However it could
> > > be racy
> > > because from_dst_file could have been reset right after checking it but
> > > before
> > > passing it to qemu_file_shutdown() by the rp_thread.
> > >
> > > Fix it by shutdown() on the src file instead. Since they must be a pair
> > > of
> > > qemu files, shutdown on either of them will work the same.
> > >
> > > Since at it, drop the check for from_dst_file directly, which makes the
> > > behavior even more predictable.
> >
> > So while the existing code maybe racy, I'm not sure that this change
> > keeps the semantics; the channel may well have dup()'d the fd's for the
> > two directions, and I'm not convinced that a shutdown() on one will
> > necessarily impact the other; and if the shutdown doesn't happen the
> > rp_thread might not exit, and we might block on the koin.
>
> dup() seems fine as long as the backend file/socket is the same; but I get the
> point here, e.g., potentially from/to channels can indeed be different ones.
>
> >
> > Why don't we solve this a different way - how about we move the:
> > ms->rp_state.from_dst_file = NULL;
> > qemu_fclose(rp);
> >
> > out of the source_return_path_thread and put it in
> > await_return_path_close_on_source, immediately after the join?
> > Then we *know* that the the rp thread isn't messing with it.
>
> Yes that looks working for this special case of when rp_thread quits.
>
> It's just that it'll make things a bit more complicated, previously
> from_dst_file is only reset in rp_thread (for either paused or completed
> migration), now we handle "migration completes" a bit different.
>
> This also reminded me that maybe we can also use the qemu_file_lock mutex as
> we
> use to try protect access to to_dst_file, because I think from_dst_file is
> potentially racy in the same way. Even if we moved the reset to migration
> thread, we still have e.g. migrate_fd_cancel() calling:
>
> if (s->rp_state.from_dst_file) {
> qemu_file_shutdown(s->rp_state.from_dst_file);
> }
>
> I think that is also racy too when running in the main thread, as either the
> migration thread or rp_thread could have reset it right after the check.
>
> So.. maybe I start to use the qemu_file_lock to cover from_dst_file cases too?
> Then I'll cover at leasd both migrate_fd_cancel() and this case.
Yes, I think that's best; but try to do as little as possible with the
lock held.
Dave
> Thanks,
>
> --
> Peter Xu
>
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
[PATCH 3/5] migration: Introduce migration_ioc_[un]register_yank(), Peter Xu, 2021/07/20
[PATCH 4/5] migration: Teach QEMUFile to be QIOChannel-aware, Peter Xu, 2021/07/20
[PATCH 5/5] migration: Move the yank unregister of channel_close out, Peter Xu, 2021/07/20