Re: [PATCH 3/3] migration/postcopy: handle POSTCOPY_INCOMING_RUNNING corner case properly

[Wei Yang]

On Wed, Oct 09, 2019 at 01:36:34PM +0800, Peter Xu wrote:
>On Wed, Oct 09, 2019 at 01:07:56PM +0800, Wei Yang wrote:
>> On Wed, Oct 09, 2019 at 12:12:25PM +0800, Peter Xu wrote:
>> >On Wed, Oct 09, 2019 at 09:02:04AM +0800, Wei Yang wrote:
>> >> On Tue, Oct 08, 2019 at 05:40:46PM +0100, Dr. David Alan Gilbert wrote:
>> >> >* 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 introducing an old_state parameter in 
>> >> >> postcopy_state_set().
>> >> >> New state only would be set when current state equals to old_state.
>> >> >> 
>> >> >> Signed-off-by: Wei Yang <address@hidden>
>> >> >
>> >> >OK, it's a shame to use a pointer there, but it works.
>> >> 
>> >> You mean second parameter of postcopy_state_set()?
>> >> 
>> >> I don't have a better idea. Or we introduce a new state
>> >> POSTCOPY_INCOMING_NOCHECK. Do you feel better with this?
>> >
>> >Maybe simply fix loadvm_postcopy_handle_run() to set the state after
>> >the POSTCOPY_INCOMING_LISTENING check?
>> >
>> 
>> Set state back to ps if ps is not POSTCOPY_INCOMING_LISTENING?
>> 
>> Sounds like another option.
>
>Even simpler?
>
>  ps = postcopy_state_get();
>  if (ps != INCOMING)
>    return -1;
>  postcopy_state_set(RUNNING);
>

Looks good to me.

Dave,

Do you feel good with it?

>Thanks,
>
>-- 
>Peter Xu

-- 
Wei Yang
Help you, Help me