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

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Date:	Wed, 4 Nov 2009 13:57:29 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Gregory Haskins <gregory.haskins@...il.com>,
	Eric Dumazet <eric.dumazet@...il.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
Subject: Re: [PATCHv7 3/3] vhost_net: a kernel-level virtio server

On Tue, Nov 03, 2009 at 03:57:44PM -0800, Paul E. McKenney wrote:
> On Tue, Nov 03, 2009 at 01:14:06PM -0500, Gregory Haskins wrote:
> > 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
> 
> What was happening in that case was that the rcu_dereference()
> was being used in a workqueue item.  The role of rcu_read_lock()
> was taken on be the start of execution of the workqueue item, of
> rcu_read_unlock() by the end of execution of the workqueue item, and
> of synchronize_rcu() by flush_workqueue().  This does work, at least
> assuming that flush_workqueue() operates as advertised, which it appears
> to at first glance.
> 
> The above code looks somewhat different, however -- I don't see
> handle_tx() being executed in the context of a work queue.  Instead
> it appears to be in an interrupt handler.
> So what is the story?  Using synchronize_irq() or some such?
> 
> 							Thanx, Paul

No, there has been no change (I won't be able to use a mutex in an
interrupt handler, will I?).  handle_tx is still called in the context
of a work queue: either from handle_tx_kick or from handle_tx_net which
are work queue items.

Can you ack this usage please?

-- 
MST
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