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Re: [Qemu-devel] [RFC 5/6] migration: store listen task tag


From: Daniel P. Berrange
Subject: Re: [Qemu-devel] [RFC 5/6] migration: store listen task tag
Date: Tue, 15 Aug 2017 09:37:14 +0100
User-agent: Mutt/1.8.3 (2017-05-23)

On Tue, Aug 15, 2017 at 02:17:06PM +0800, Peter Xu wrote:
> Store the task tag for migration types: tcp/unix/fd/exec in current
> MigrationIncomingState struct.
> 
> For defered migration, no need to store task tag since there is no task
> running in the main loop at all. For RDMA, let's mark it as todo.
> 
> Signed-off-by: Peter Xu <address@hidden>
> ---
>  migration/migration.c | 22 ++++++++++++++++++----
>  migration/migration.h |  2 ++
>  2 files changed, 20 insertions(+), 4 deletions(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index c9b7085..daf356b 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -171,6 +171,7 @@ void migration_incoming_state_destroy(void)
>          mis->from_src_file = NULL;
>      }
>  
> +    mis->listen_task_tag = 0;
>      qemu_event_destroy(&mis->main_thread_load_event);
>  }
>  
> @@ -265,25 +266,31 @@ int migrate_send_rp_req_pages(MigrationIncomingState 
> *mis, const char *rbname,
>  void qemu_start_incoming_migration(const char *uri, Error **errp)
>  {
>      const char *p;
> +    guint task_tag = 0;
> +    MigrationIncomingState *mis = migration_incoming_get_current();
>  
>      qapi_event_send_migration(MIGRATION_STATUS_SETUP, &error_abort);
>      if (!strcmp(uri, "defer")) {
>          deferred_incoming_migration(errp);
>      } else if (strstart(uri, "tcp:", &p)) {
> -        tcp_start_incoming_migration(p, errp);
> +        task_tag = tcp_start_incoming_migration(p, errp);
>  #ifdef CONFIG_RDMA
>      } else if (strstart(uri, "rdma:", &p)) {
> +        /* TODO: store task tag for RDMA migrations */
>          rdma_start_incoming_migration(p, errp);
>  #endif
>      } else if (strstart(uri, "exec:", &p)) {
> -        exec_start_incoming_migration(p, errp);
> +        task_tag = exec_start_incoming_migration(p, errp);
>      } else if (strstart(uri, "unix:", &p)) {
> -        unix_start_incoming_migration(p, errp);
> +        task_tag = unix_start_incoming_migration(p, errp);
>      } else if (strstart(uri, "fd:", &p)) {
> -        fd_start_incoming_migration(p, errp);
> +        task_tag = fd_start_incoming_migration(p, errp);
>      } else {
>          error_setg(errp, "unknown migration protocol: %s", uri);
> +        return;
>      }
> +
> +    mis->listen_task_tag = task_tag;

This is unsafe as currently written. The main loop watches that are
associated with these tags are all unregistered when incoming migration
starts. So if you save them, and then unregister later when postcopy
is "stuck", then you're likely unregistering a tag associated with
some other random part of QEMU, because the saved value is no longer
valid.


Regards,
Daniel
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