Re: [Qemu-devel] [PATCH v5 17/45] Add wrappers and handlers for sending/receiving the postcopy-ram migration messages.

[Paolo Bonzini]

On 31/03/2015 13:05, Dr. David Alan Gilbert wrote:
> * Paolo Bonzini (address@hidden) wrote:
>>
>>
>> On 30/03/2015 19:46, Dr. David Alan Gilbert wrote:
>>>>>>> +PostcopyState  postcopy_state_get(MigrationIncomingState *mis)
>>>>>>> +{
>>>>>>> +    return atomic_fetch_add(&mis->postcopy_state, 0);
>>>>>>> +}
>>>>>>> +
>>>>>>> +/* Set the state and return the old state */
>>>>>>> +PostcopyState postcopy_state_set(MigrationIncomingState *mis,
>>>>>>> +                                 PostcopyState new_state)
>>>>>>> +{
>>>>>>> +    return atomic_xchg(&mis->postcopy_state, new_state);
>>>>>>> +}
>>>>>
>>>>> Which are the (multiple) threads are calling these functions?
>>> The main thread receiving the migration and the postcopy ram_listen_thread
>>> receiving the RAM pages.
>>> It's not actually racy between multiple threads updating it,
>>> it's sequenced so that the main thread initialises it and then
>>> hands over to the listen thread that takes it over from that point.
>>
>> I would use atomic_mb_read/atomic_mb_set here.
> 
> The benefit of the atomic_xchg is that I get the old value back so
> that I can sanity check I was in the state I thought I should have
> been.

Ok, you can still pair atomic_xchg with atomic_mb_read, it's a bit cheaper.

Paolo