From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCHv7 3/3] vhost_net: a kernel-level virtio server Date: Tue, 3 Nov 2009 15:57:44 -0800 Message-ID: <20091103235744.GF6726@linux.vnet.ibm.com> References: <20091103172422.GD5591@redhat.com> <4AF0708B.4020406@gmail.com> <4AF07199.2020601@gmail.com> <4AF072EE.9020202@gmail.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: quoted-printable Cc: Eric Dumazet , "Michael S. Tsirkin" , netdev@vger.kernel.org, virtualization@lists.linux-foundation.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, mingo@elte.hu, linux-mm@kvack.org, akpm@linux-foundation.org, hpa@zytor.com, Rusty Russell , s.hetze@linux-ag.com To: Gregory Haskins Return-path: Content-Disposition: inline In-Reply-To: <4AF072EE.9020202@gmail.com> Sender: owner-linux-mm@kvack.org List-Id: netdev.vger.kernel.org On Tue, Nov 03, 2009 at 01:14:06PM -0500, Gregory Haskins wrote: > Gregory Haskins wrote: > > Eric Dumazet wrote: > >> Michael S. Tsirkin a =E9crit : > >>> +static void handle_tx(struct vhost_net *net) > >>> +{ > >>> + struct vhost_virtqueue *vq =3D &net->dev.vqs[VHOST_NET_VQ_TX]; > >>> + unsigned head, out, in, s; > >>> + struct msghdr msg =3D { > >>> + .msg_name =3D NULL, > >>> + .msg_namelen =3D 0, > >>> + .msg_control =3D NULL, > >>> + .msg_controllen =3D 0, > >>> + .msg_iov =3D vq->iov, > >>> + .msg_flags =3D MSG_DONTWAIT, > >>> + }; > >>> + size_t len, total_len =3D 0; > >>> + int err, wmem; > >>> + size_t hdr_size; > >>> + struct socket *sock =3D rcu_dereference(vq->private_data); > >>> + if (!sock) > >>> + return; > >>> + > >>> + wmem =3D atomic_read(&sock->sk->sk_wmem_alloc); > >>> + if (wmem >=3D sock->sk->sk_sndbuf) > >>> + return; > >>> + > >>> + use_mm(net->dev.mm); > >>> + mutex_lock(&vq->mutex); > >>> + vhost_no_notify(vq); > >>> + > >> using rcu_dereference() and mutex_lock() at the same time seems wron= g, I suspect > >> that your use of RCU is not correct. > >> > >> 1) rcu_dereference() should be done inside a read_rcu_lock() section= , and > >> we are not allowed to sleep in such a section. > >> (Quoting Documentation/RCU/whatisRCU.txt : > >> It is illegal to block while in an RCU read-side critical secti= on, ) > >> > >> 2) mutex_lock() can sleep (ie block) > >> > >=20 > >=20 > > Michael, > > I warned you that this needed better documentation ;) > >=20 > > Eric, > > I think I flagged this once before, but Michael convinced me that i= t > > was indeed "ok", if but perhaps a bit unconventional. I will try to > > find the thread. > >=20 > > Kind Regards, > > -Greg > >=20 >=20 > Here it is: >=20 > http://lkml.org/lkml/2009/8/12/173 What was happening in that case was that the rcu_dereference() was being used in a workqueue item. The role of rcu_read_lock() was taken on be the start of execution of the workqueue item, of rcu_read_unlock() by the end of execution of the workqueue item, and of synchronize_rcu() by flush_workqueue(). This does work, at least assuming that flush_workqueue() operates as advertised, which it appears to at first glance. The above code looks somewhat different, however -- I don't see handle_tx() being executed in the context of a work queue. Instead it appears to be in an interrupt handler. So what is the story? Using synchronize_irq() or some such? Thanx, Paul -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org