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Message-ID: <4AF07BB7.1020802@gmail.com>
Date: Tue, 03 Nov 2009 19:51:35 +0100
From: Eric Dumazet <eric.dumazet@...il.com>
To: Gregory Haskins <gregory.haskins@...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
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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