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 13:28:31 +0300

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?

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'?

> > 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;
> > >