Re: [Qemu-devel] [PULL 27/28] migration: protect migration_bitmap

[Wen Congyang <wency@cn.fujitsu.com>]

On 07/09/2015 03:59 PM, Paolo Bonzini wrote:
> 
> 
> On 09/07/2015 03:19, Wen Congyang wrote:
>> Yes, why migration doesn't trigger this problem? We will fix it soon.
> 
> This should be the fix if someone wants to test it and/or post it:
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index 57368e1..8d5a73a 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -1227,9 +1227,9 @@ static int ram_save_complete(QEMUFile *f, void
> *opaque)
> 
>      flush_compressed_data(f);
>      ram_control_after_iterate(f, RAM_CONTROL_FINISH);
> -    migration_end();
> -
>      rcu_read_unlock();
> +
> +    migration_end();
>      qemu_put_be64(f, RAM_SAVE_FLAG_EOS);

Yes, this patch can work. But if the caller hold the rcu read lock in
the future, we may need to fix it again. I think it is better to use
call_rcu().

> 
>      return 0;
> 
> 
> You don't see it with migration because the migration thread (and for
> that matter, _no_ thread except the I/O thread!) is not registered with
> the RCU subsystem.  I'm working on it, but I plan to only fix it in
> later release candidates.

Thanks for the explantion.

Wen Congyang

> 
> Paolo
>