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Message-ID: <4AF07199.2020601@gmail.com>
Date: Tue, 03 Nov 2009 13:08:25 -0500
From: Gregory Haskins <gregory.haskins@gmail.com>
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To: Eric Dumazet <eric.dumazet@gmail.com>
CC: "Michael S. Tsirkin" <mst@redhat.com>, 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 <rusty@rustcorp.com.au>, s.hetze@linux-ag.com,
       "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: [PATCHv7 3/3] vhost_net: a kernel-level virtio server
References: <cover.1257267892.git.mst@redhat.com> <20091103172422.GD5591@redhat.com> <4AF0708B.4020406@gmail.com>
In-Reply-To: <4AF0708B.4020406@gmail.com>
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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);
>> +
>=20
> using rcu_dereference() and mutex_lock() at the same time seems wrong, =
I suspect
> that your use of RCU is not correct.
>=20
> 1) rcu_dereference() should be done inside a read_rcu_lock() section, a=
nd
>    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,=
 )
>=20
> 2) mutex_lock() can sleep (ie block)
>=20


Michael,
  I warned you that this needed better documentation ;)

Eric,
  I think I flagged this once before, but Michael convinced me that it
was indeed "ok", if but perhaps a bit unconventional.  I will try to
find the thread.

Kind Regards,
-Greg


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