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

Eric Dumazet
2009-11-03T18:52:16Z

Gregory Haskins a écrit :
> 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
>

Yes, this doesnt convince me at all, and could be a precedent for a wrong RCU use.
People wanting to use RCU do a grep on kernel sources to find how to correctly
use RCU.

Michael, please use existing locking/barrier mechanisms, and not pretend to use RCU.

Some automatic tools might barf later.

For example, we could add a debugging facility to check that rcu_dereference() is used
in an appropriate context, ie conflict with existing mutex_lock() debugging facility.