Re: [PATCH v2 1/5] migration/postcopy: report COMPLETED only after the run BH has run

[Eric Blake <eblake@redhat.com>]

On Tue, Nov 10, 2026 at 07:32:53PM +0100, Denis V. Lunev wrote:
> From: Denis V. Lunev <den@openvz.org>
> 
> The postcopy listen thread waits for main_thread_load_event before it
> reports COMPLETED. That event is set as soon as qemu_loadvm_state_main()
> returns in the main thread, while the run state is changed later, in
> loadvm_postcopy_handle_run_bh(). The listen thread can therefore report
> COMPLETED while the VM is still in RUN_STATE_INMIGRATE, easily so in a
> bitmap-only postcopy where it has little to load.
> 
> Management reacting to COMPLETED then finds the VM still incoming: a
> blockdev-add creates an inactive node, since bds_tree_init() forces
> BDRV_O_INACTIVE in RUN_STATE_INMIGRATE, and attaching a device to it
> fails with "Block node is read-only". Precopy gets this right:
> process_incoming_migration_bh() changes the run state before it reports
> COMPLETED.
> 
> Once CMD_POSTCOPY_RUN has scheduled the run BH, set the event at the
> end of the BH instead. qemu_loadvm_state() keeps setting it in every
> other case, including a load that failed before CMD_POSTCOPY_RUN. The
> postcopy state it checks only becomes RUNNING on the main thread, right
> before the BH is scheduled.
> 
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: Fabiano Rosas <farosas@suse.de>
> Cc: Juraj Marcin <jmarcin@redhat.com>
> ---
>  migration/migration.h | 2 +-
>  migration/savevm.c    | 6 +++++-
>  2 files changed, 6 insertions(+), 2 deletions(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org