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

From: Eric Dumazet <[email-masked]>
Date: Tue, 03 Nov 2009 19:51:35 +0100

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.

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