From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH net-next]vhost: fix condition check for # of outstanding dma buffers Date: Wed, 20 Jul 2011 19:17:21 +0300 Message-ID: <20110720161721.GD8077@redhat.com> References: <1311100678.8573.16.camel@localhost.localdomain> <20110719190945.GB8667@redhat.com> <1311108985.8573.30.camel@localhost.localdomain> <20110720102831.GA5164@redhat.com> <1311176589.8573.33.camel@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: David Miller , netdev@vger.kernel.org, jasowang@redhat.com To: Shirley Ma Return-path: Received: from mx1.redhat.com ([209.132.183.28]:34940 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752397Ab1GTQRD (ORCPT ); Wed, 20 Jul 2011 12:17:03 -0400 Content-Disposition: inline In-Reply-To: <1311176589.8573.33.camel@localhost.localdomain> Sender: netdev-owner@vger.kernel.org List-ID: 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 > > > > > --- > > > > > > > > > > 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