[Qemu-devel] lock-free monitor?

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

Hi,
  I wondered what it would take to be able to do a lock-free monitor;
i.e. one that could respond to (some) commands while the qemu big lock is held.

My reason for asking is that there are cases in migration and colo
where a network block device has failed and is blocking, but it fails
at just the wrong time while the lock is held, and now, you don't have
any way to unblock it since you can't do anything on the monitors.
If I could issue a command then I could have it end up doing a shutdown(2)
on the network connection and free things up.

Maybe this would also help real-time people?

I was thinking then, we could:
   a) Have a monitor that wasn't tied to the main loop at all - each instance
would be it's own thread and have it's own poll() (probably using the new
IO channels?)
   b) for most commands it would take the big lock before execute the command
and release it afterwards - so there would be no risk in those commands.
   c) Some commands you could mark as 'lock free' - they would be required
not to take any locks or wait for *anything* and for those the monitor would
not take the lock.
   d) Perhaps have some way of restricting a particular monitor session to only
allowing non-blocking commands.
   e) Then I'd have to have a way of finding the broken device in a lockfree
way (RTU list walk?) and issuing the shutdown(2).

Bonus points to anyone who can statically check commands in (c).

Does this make sense to everyone else, or does anyone have any better
suggestions?

Dave

--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK