Re: [PATCH 0/2] Postcopy migration and vhost-user errors

[Peter Xu]

Hi, Michael,

On Wed, Jul 17, 2024 at 04:55:52AM -0400, Michael S. Tsirkin wrote:
> I just want to understand how we managed to have two threads
> talking in parallel. BQL is normally enough, which path
> manages to invoke vhost-user with BQL not taken?
> Just check BQL taken on each vhost user invocation and
> you will figure it out.

Prasad mentioned how the race happened in the cover letter:

20240711131424.181615-1-ppandit@redhat.com">https://lore.kernel.org/r/20240711131424.181615-1-ppandit@redhat.com

     Thread-1                                  Thread-2

vhost_dev_start                        postcopy_ram_incoming_cleanup
 vhost_device_iotlb_miss                postcopy_notify
  vhost_backend_update_device_iotlb      vhost_user_postcopy_notifier
   vhost_user_send_device_iotlb_msg       vhost_user_postcopy_end
    process_message_reply                  process_message_reply
     vhost_user_read                        vhost_user_read
      vhost_user_read_header                 vhost_user_read_header
       "Fail to update device iotlb"          "Failed to receive reply to 
postcopy_end"

The normal case should be that thread-2 is postcopy_ram_listen_thread(),
and this happens when postcopy migration is close to the end.

Thanks,

-- 
Peter Xu