Re: [RFC PATCH 3/4] migration: Make listen thread joinable

[Peter Xu <peterx@redhat.com>]

On Thu, Aug 07, 2025 at 01:49:11PM +0200, Juraj Marcin wrote:
> From: Juraj Marcin <jmarcin@redhat.com>
> 
> This patch allows joining the migration listen thread. This is done in
> preparation for the introduction of "postcopy-setup" state at the
> beginning of a postcopy migration, when destination can fail gracefully
> and source side then resume to a running state.
> 
> In case of such failure, to gracefully perform all cleanup in the main
> migration thread, we need to wait for the listen thread to exit, which
> can be done by joining it.
> 
> Signed-off-by: Juraj Marcin <jmarcin@redhat.com>
> ---
>  migration/migration.c | 1 +
>  migration/savevm.c    | 2 +-
>  2 files changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index e5ce2940d5..8174e811eb 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -901,6 +901,7 @@ process_incoming_migration_co(void *opaque)
>               * Postcopy was started, cleanup should happen at the end of the
>               * postcopy thread.
>               */
> +            qemu_thread_detach(&mis->listen_thread);
>              trace_process_incoming_migration_co_postcopy_end_main();
>              goto out;
>          }
> diff --git a/migration/savevm.c b/migration/savevm.c
> index fabbeb296a..d2360be53c 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -2217,7 +2217,7 @@ static int loadvm_postcopy_handle_listen(MigrationIncomingState *mis)
>      mis->have_listen_thread = true;
>      postcopy_thread_create(mis, &mis->listen_thread,
>                             MIGRATION_THREAD_DST_LISTEN,
> -                           postcopy_ram_listen_thread, QEMU_THREAD_DETACHED);
> +                           postcopy_ram_listen_thread, QEMU_THREAD_JOINABLE);

This is good; I actually forgot it used to be detached..

Instead of proactively detach it above, could we always properly join it
(and hopefully every migration thread)?  Then we could drop patch 1 too.

>      trace_loadvm_postcopy_handle_listen("return");
>  
>      return 0;
> -- 
> 2.50.1
> 

-- 
Peter Xu