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

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Date:	Tue, 03 Nov 2009 13:08:25 -0500
From:	Gregory Haskins <gregory.haskins@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	"Michael S. Tsirkin" <mst@...hat.com>, netdev@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, mingo@...e.hu, linux-mm@...ck.org,
	akpm@...ux-foundation.org, hpa@...or.com,
	Rusty Russell <rusty@...tcorp.com.au>, s.hetze@...ux-ag.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCHv7 3/3] vhost_net: a kernel-level virtio server

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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