From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH net V2 2/4] vhost_net: switch to use mutex_trylock() in vhost_net_busy_poll() Date: Wed, 12 Dec 2018 09:20:57 -0500 Message-ID: <20181212091957-mutt-send-email-mst@kernel.org> References: <20181212100819.21295-1-jasowang@redhat.com> <20181212100819.21295-3-jasowang@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Tonghao Zhang , David Miller To: Jason Wang Return-path: Content-Disposition: inline In-Reply-To: <20181212100819.21295-3-jasowang@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Wed, Dec 12, 2018 at 06:08:17PM +0800, Jason Wang wrote: > We used to hold the mutex of paired virtqueue in > vhost_net_busy_poll(). But this will results an inconsistent lock > order which may cause deadlock if we try to bring back the protection > of device IOTLB with vq mutex that requires to hold mutex of all > virtqueues at the same time. > > Fix this simply by switching to use mutex_trylock(), when fail just > skip the busy polling. This can happen when device IOTLB is under > updating which should be rare. > > Fixes: commit 78139c94dc8c ("net: vhost: lock the vqs one by one") > Cc: Tonghao Zhang > Signed-off-by: Jason Wang Acked-by: Michael S. Tsirkin and I think we should try to put this fix in 4.20 too. > --- > drivers/vhost/net.c | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c > index ab11b2bee273..ad7a6f475a44 100644 > --- a/drivers/vhost/net.c > +++ b/drivers/vhost/net.c > @@ -513,7 +513,13 @@ static void vhost_net_busy_poll(struct vhost_net *net, > struct socket *sock; > struct vhost_virtqueue *vq = poll_rx ? tvq : rvq; > > - mutex_lock_nested(&vq->mutex, poll_rx ? VHOST_NET_VQ_TX: VHOST_NET_VQ_RX); > + /* Try to hold the vq mutex of the paired virtqueue. We can't > + * use mutex_lock() here since we could not guarantee a > + * consistenet lock ordering. > + */ > + if (!mutex_trylock(&vq->mutex)) > + return; > + > vhost_disable_notify(&net->dev, vq); > sock = rvq->private_data; > > -- > 2.17.1