Subject: Re: [PATCH net-next]vhost: fix condition check for # of outstanding dma buffers

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 20 Jul 2011 19:17:21 +0300

On Wed, Jul 20, 2011 at 08:43:09AM -0700, Shirley Ma wrote:
> On Wed, 2011-07-20 at 13:28 +0300, Michael S. Tsirkin wrote:
> > On Tue, Jul 19, 2011 at 01:56:25PM -0700, Shirley Ma wrote:
> > > On Tue, 2011-07-19 at 22:09 +0300, Michael S. Tsirkin wrote:
> > > > On Tue, Jul 19, 2011 at 11:37:58AM -0700, Shirley Ma wrote:
> > > > > Signed-off-by: Shirley Ma <[email-masked]>
> > > > > ---
> > > > > 
> > > > >  drivers/vhost/net.c |    6 ++++--
> > > > >  1 files changed, 4 insertions(+), 2 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > > > > index 70ac604..83cb738 100644
> > > > > --- a/drivers/vhost/net.c
> > > > > +++ b/drivers/vhost/net.c
> > > > > @@ -189,8 +189,10 @@ static void handle_tx(struct vhost_net
> > *net)
> > > > >                               break;
> > > > >                       }
> > > > >                       /* If more outstanding DMAs, queue the
> > work */
> > > > > -                     if (unlikely(vq->upend_idx - vq->done_idx
> > >
> > > > > -                                  VHOST_MAX_PEND)) {
> > > > > +                     if (unlikely((vq->upend_idx - vq->done_idx
> > >
> > > > > +                                     VHOST_MAX_PEND) ||
> > > > > +                                  (vq->upend_idx - vq->done_idx
> > >
> > > > > +                                      VHOST_MAX_PEND -
> > > > UIO_MAXIOV))) {
> > > > 
> > > > Could you please explain why this makes sense please?
> > > > VHOST_MAX_PEND is 128 UIO_MAXIOV is 1024 so
> > > > the result is negative?
> > > 
> > > I thought it is equal to:
> > > 
> > > if (vq->upend_idx > vq->done_idx) 
> > >       check vq->upend_idx - vq->done_idx > VHOST_MAX_PEND
> > > if (vq->upend_idx < vq->done_idx)
> > >       check vq->upend_idx + UIO_MAXIOV - vq->done_idx >
> > VHOST_MAX_PEND
> > >       
> > 
> > Check it out: upend_idx == done_idx == 0 does not satisfy the
> > above conditions but does trigger in your code, right?
> 
> We don't hit upend_idx == done_idx == 0. Only upend_idx == done_idx ==
> UIO_MAXIOV could happen if the lower device has issue and never DMA any
> packets out.

My point was that the logic isn't the same, even though
you said 'it is equal to'.
Same applies to upend_idx == 1, done_idx == 0.

> > Better keep it simple. Maybe:
> > 
> >         if (unlikely(vq->upend_idx - vq->done_idx > VHOST_MAX_PEND) ||
> >                 (unlikely(vq->upend_idx < vq->done_idx) &&
> >                 unlikely(vq->upend_idx + UIO_MAXIOV - vq->done_idx >
> >                          VHOST_MAX_PEND)))
> > 
> > ?
> > 
> > Also, please add commit log documenting what does the patch
> > fix: something like:
> >         'the test for # of outstanding buffers returned
> >          incorrect results when due to wrap around,
> >          upend_idx < done_idx'?
> 
> Sure, will modify it and resubmit.
> 
> > > > I thought upend_idx - done_idx is exactly the number
> > > > of buffers, so once we get too many we stop until
> > > > one gets freed?
> > > 
> > > They are index, so in vhost zerocopy callback, we can get the idx
> > right
> > > away.
> > > 
> > > > 
> > > > >                               tx_poll_start(net, sock);
> > > > >                               set_bit(SOCK_ASYNC_NOSPACE,
> > > > &sock->flags);
> > > > >                               break;
> > > > > 
> 
> Thanks
> Shirley