netdev - Re: [PATCHv7 3/3] vhost_net: a kernel-level virtio server

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Message-ID: <20091103195557.GA6669@redhat.com>
Date:	Tue, 3 Nov 2009 21:55:57 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, mingo@...e.hu,
	linux-mm@...ck.org, akpm@...ux-foundation.org, hpa@...or.com,
	gregory.haskins@...il.com, Rusty Russell <rusty@...tcorp.com.au>,
	s.hetze@...ux-ag.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCHv7 3/3] vhost_net: a kernel-level virtio server

On Tue, Nov 03, 2009 at 07:03:55PM +0100, Eric Dumazet wrote:
> Michael S. Tsirkin a écrit :
> > +static void handle_tx(struct vhost_net *net)
> > +{
> > +	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
> > +	unsigned head, out, in, s;
> > +	struct msghdr msg = {
> > +		.msg_name = NULL,
> > +		.msg_namelen = 0,
> > +		.msg_control = NULL,
> > +		.msg_controllen = 0,
> > +		.msg_iov = vq->iov,
> > +		.msg_flags = MSG_DONTWAIT,
> > +	};
> > +	size_t len, total_len = 0;
> > +	int err, wmem;
> > +	size_t hdr_size;
> > +	struct socket *sock = rcu_dereference(vq->private_data);
> > +	if (!sock)
> > +		return;
> > +
> > +	wmem = atomic_read(&sock->sk->sk_wmem_alloc);
> > +	if (wmem >= 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 wrong, 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 section, )
> 
> 2) mutex_lock() can sleep (ie block)

This use is correct. See comment in vhost.h This use of RCU has been
acked by Paul E. McKenney (paulmck@...ux.vnet.ibm.com) as well.
There are many ways to use RCU not all of which involve read_rcu_lock.

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