Re: [Qemu-devel] [PATCH 2/2] migration: move bdrv_invalidate_cache_all of of coroutine context

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

* Denis V. Lunev (den@openvz.org) wrote:
> There is a possibility to hit an assert in qcow2_get_specific_info that
> s->qcow_version is undefined. This happens when VM in starting from
> suspended state, i.e. it processes incoming migration, and in the same
> time 'info block' is called.
> 
> The problem is that qcow2_invalidate_cache() closes the image and
> memset()s BDRVQcowState in the middle.
> 
> The patch moves processing of bdrv_invalidate_cache_all out of
> coroutine context for postcopy migration to avoid that. This function
> is called with the following stack:
>   process_incoming_migration_co
>   qemu_loadvm_state
>   qemu_loadvm_state_main
>   loadvm_process_command
>   loadvm_postcopy_handle_run
> 
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> CC: Paolo Bonzini <pbonzini@redhat.com>
> CC: Juan Quintela <quintela@redhat.com>
> CC: Amit Shah <amit.shah@redhat.com>

I've tested my normal postcopy smoke test and it seems to work,
so
Tested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

however, can you add a comment before loadvm_postcopy_handle_run_bh
to explain why it's a bh, otherwise at some point someone might
put it back.

I'll admit to not really understanding what the difference is
between bh and coroutine context; I'd thought if it was all
in the main thread stuff was safe.

Dave

> ---
>  migration/savevm.c | 27 +++++++++++++++++----------
>  1 file changed, 17 insertions(+), 10 deletions(-)
> 
> diff --git a/migration/savevm.c b/migration/savevm.c
> index 94f2894..8415fd9 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -1496,18 +1496,10 @@ static int loadvm_postcopy_handle_listen(MigrationIncomingState *mis)
>      return 0;
>  }
>  
> -/* After all discards we can start running and asking for pages */
> -static int loadvm_postcopy_handle_run(MigrationIncomingState *mis)
> +static void loadvm_postcopy_handle_run_bh(void *opaque)
>  {
> -    PostcopyState ps = postcopy_state_set(POSTCOPY_INCOMING_RUNNING);
>      Error *local_err = NULL;
>  
> -    trace_loadvm_postcopy_handle_run();
> -    if (ps != POSTCOPY_INCOMING_LISTENING) {
> -        error_report("CMD_POSTCOPY_RUN in wrong postcopy state (%d)", ps);
> -        return -1;
> -    }
> -
>      /* TODO we should move all of this lot into postcopy_ram.c or a shared code
>       * in migration.c
>       */
> @@ -1519,7 +1511,6 @@ static int loadvm_postcopy_handle_run(MigrationIncomingState *mis)
>      bdrv_invalidate_cache_all(&local_err);
>      if (local_err) {
>          error_report_err(local_err);
> -        return -1;
>      }
>  
>      trace_loadvm_postcopy_handle_run_cpu_sync();
> @@ -1534,6 +1525,22 @@ static int loadvm_postcopy_handle_run(MigrationIncomingState *mis)
>          /* leave it paused and let management decide when to start the CPU */
>          runstate_set(RUN_STATE_PAUSED);
>      }
> +}
> +
> +/* After all discards we can start running and asking for pages */
> +static int loadvm_postcopy_handle_run(MigrationIncomingState *mis)
> +{
> +    PostcopyState ps = postcopy_state_set(POSTCOPY_INCOMING_RUNNING);
> +    QEMUBH *bh;
> +
> +    trace_loadvm_postcopy_handle_run();
> +    if (ps != POSTCOPY_INCOMING_LISTENING) {
> +        error_report("CMD_POSTCOPY_RUN in wrong postcopy state (%d)", ps);
> +        return -1;
> +    }
> +
> +    bh = qemu_bh_new(loadvm_postcopy_handle_run_bh, NULL);
> +    qemu_bh_schedule(bh);
>  
>      /* We need to finish reading the stream from the package
>       * and also stop reading anything more from the stream that loaded the
> -- 
> 2.1.4
> 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK