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

Gregory Haskins
2009-11-03T18:08:29Z

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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

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