Re: [virtio-dev] [PATCH RFC 3/3] rng: leak detection support

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

On Mon, Sep 18, 2023 at 03:00:43PM +0200, Babis Chalios wrote:
> 
> > > Right, so I think that there is a race condition between the time the driver
> > > sees the used buffers of the first
> > > batch and until it adds the second batch on the next leak queue.
> > > 
> > > 1. driver adds batch 1
> > > 2. leak event
> > > 3. device uses batch 1
> > > 4. driver sees the used buffers and
> > >      a. switches leak queues
> > >      b. adds batch 2.
> > > 5. devices finds initial leak queue empty and sees buffers in second leak
> > > queue.
> > > 
> > > If a second leak event happens after step 3 above and before all of steps 4
> > > complete then batch 2 will not
> > > be processed as part of the second leak event.
> > driver can just pre-add buffers in the second queue.
> > 
> > 1. available buffers to queue 1-X
> > 2. available buffers to queue X
> > 
> > 
> > 3. poll queue X
> > 4. used buffers in queue X
> > 5. avail buffers in queue X
> > 6. poll queue 1-X
> > 7. used buffers in queue X
> > 8. avail buffers in queue X
> > 9. goto 3
> > 
> 
> Yes, that's what the driver does now in the RFC patch. However, this just
> decreases
> the race window, it doesn't eliminate it. If a third leak event happens it
> might not
> find any buffers to use:
> 
> 1. available buffers to queue 1-X
> 2. available buffers to queue X
> 
> 
> 3. poll queue X
> 4. used buffers in queue X       <- leak event 1 will use buffers in X
> 5. avail buffers in queue X
> 6. poll queue 1-X                <- leak event 2 will use buffers in 1-X
> 7. used buffers in queue 1-X
> 8. avail buffers in queue 1-X
>                                  <- leak event 3 (it needs buffers in X, race with step 5)
> 9. goto 3


I don't get it. we added buffers in step 5.

> 
> 
> If, instead, we define a single leak queue and require that VMM should refuse to take a snapshot
> if that queue is empty, we avoid the race condition in all cases and IMHO the protocol becomes
> much simpler.
> 
> 
> Cheers,
> Babis
>