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

[Wei Yang <richardw.yang@linux.intel.com>]

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 (richardw.yang@linux.intel.com) 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 <richardw.yang@linux.intel.com>
>> >
>> >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.


-- 
Wei Yang
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