Re: [PATCH v2 2/2] migration/postcopy: check PostcopyState before setting to POSTCOPY_INCOMING_RUNNING

[Dr. David Alan Gilbert]

* Wei Yang (address@hidden) wrote:
> Currently, we set PostcopyState blindly to RUNNING, even we found the
> previous state is not LISTENING. This will lead to a corner case.
> 
> First let's look at the code flow:
> 
> qemu_loadvm_state_main()
>     ret = loadvm_process_command()
>         loadvm_postcopy_handle_run()
>             return -1;
>     if (ret < 0) {
>         if (postcopy_state_get() == POSTCOPY_INCOMING_RUNNING)
>             ...
>     }
> 
> >From above snippet, the corner case is loadvm_postcopy_handle_run()
> always sets state to RUNNING. And then it checks the previous state. If
> the previous state is not LISTENING, it will return -1. But at this
> moment, PostcopyState is already been set to RUNNING.
> 
> Then ret is checked in qemu_loadvm_state_main(), when it is -1
> PostcopyState is checked. Current logic would pause postcopy and retry
> if PostcopyState is RUNNING. This is not what we expect, because
> postcopy is not active yet.
> 
> This patch makes sure state is set to RUNNING only previous state is
> LISTENING by checking the state first.
> 
> Signed-off-by: Wei Yang <address@hidden>
> Suggested by: Peter Xu <address@hidden>

Reviewed-by: Dr. David Alan Gilbert <address@hidden>

> ---
>  migration/savevm.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/migration/savevm.c b/migration/savevm.c
> index 78c2965ca4..b9f30a7090 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -1934,7 +1934,7 @@ static void loadvm_postcopy_handle_run_bh(void *opaque)
>  /* 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);
> +    PostcopyState ps = postcopy_state_get();
>  
>      trace_loadvm_postcopy_handle_run();
>      if (ps != POSTCOPY_INCOMING_LISTENING) {
> @@ -1942,6 +1942,7 @@ static int 
> loadvm_postcopy_handle_run(MigrationIncomingState *mis)
>          return -1;
>      }
>  
> +    postcopy_state_set(POSTCOPY_INCOMING_RUNNING);
>      mis->bh = qemu_bh_new(loadvm_postcopy_handle_run_bh, mis);
>      qemu_bh_schedule(mis->bh);
>  
> -- 
> 2.17.1
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK