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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Nov 03, 2009 at 07:51:35PM +0100, Eric Dumazet wrote:
> Gregory Haskins a ecrit :
> > Gregory Haskins wrote:
> >> Eric Dumazet wrote:
> >>> Michael S. Tsirkin a ecrit :
> >>>> +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

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