Re: [Qemu-devel] [RFC v2 8/8] migration: add incoming mgmt lock

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

* Peter Xu (peterx@redhat.com) wrote:
> Now at least migrate_incoming can be run in parallel.  Let's provide a
> migration lock to protect it.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  migration/migration.c | 6 ++++++
>  migration/migration.h | 3 +++
>  2 files changed, 9 insertions(+)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index c3fe0ed..32058f7 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -145,6 +145,7 @@ MigrationIncomingState *migration_incoming_get_current(void)
>          mis_current.state = MIGRATION_STATUS_NONE;
>          memset(&mis_current, 0, sizeof(MigrationIncomingState));
>          qemu_mutex_init(&mis_current.rp_mutex);
> +        qemu_mutex_init(&mis_current.mgmt_mutex);
>          qemu_event_init(&mis_current.main_thread_load_event, false);
>          once = true;
>      }
> @@ -1171,6 +1172,7 @@ void qmp_migrate_incoming(const char *uri, Error **errp)
>  {
>      Error *local_err = NULL;
>      static bool once = true;
> +    MigrationIncomingState *mis = migration_incoming_get_current();

migration_incoming_get_current isn't actually thread-safe itself unless
you can guarantee the initial allocation has happened - otherwise both
threads can race and do the 'once' code at the same time.

Similarly, these locks - they don't protect our 'once' - so a second
thread could come in here and both get past the !once check.

Dave

>  
>      if (!deferred_incoming) {
>          error_setg(errp, "For use with '-incoming defer'");
> @@ -1180,8 +1182,12 @@ void qmp_migrate_incoming(const char *uri, Error **errp)
>          error_setg(errp, "The incoming migration has already been started");
>      }
>  
> +    qemu_mutex_lock(&mis->mgmt_mutex);
> +
>      qemu_start_incoming_migration(uri, &local_err);
>  
> +    qemu_mutex_unlock(&mis->mgmt_mutex);
> +
>      if (local_err) {
>          error_propagate(errp, local_err);
>          return;
> diff --git a/migration/migration.h b/migration/migration.h
> index 148c9fa..95f077b 100644
> --- a/migration/migration.h
> +++ b/migration/migration.h
> @@ -58,6 +58,9 @@ struct MigrationIncomingState {
>      /* The coroutine we should enter (back) after failover */
>      Coroutine *migration_incoming_co;
>      QemuSemaphore colo_incoming_sem;
> +
> +    /* Migration incoming management lock */
> +    QemuMutex mgmt_mutex;
>  };
>  
>  MigrationIncomingState *migration_incoming_get_current(void);
> -- 
> 2.7.4
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK