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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jul 17, 2024 at 09:33:01AM -0400, Peter Xu wrote:
> 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:
> 
> 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


OK, so postcopy_ram_ things run without the BQL?

-- 
MST