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

Gregory Haskins
2009-11-03T18:14:10Z

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Gregory Haskins wrote:
> 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)
>>
>
>
> 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
>

Here it is:

http://lkml.org/lkml/2009/8/12/173

Kind Regards,
-Greg

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