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

Michael S. Tsirkin
2009-11-03T20:02:57Z

On Tue, Nov 03, 2009 at 07:51:35PM +0100, Eric Dumazet wrote:
> 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.

Paul, you acked this previously. Should I add you acked-by line so
people calm down? If you would rather I replace
rcu_dereference/rcu_assign_pointer with rmb/wmb, I can do this.
Or maybe patch Documentation to explain this RCU usage?

--
MST