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

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Date:	Tue, 03 Nov 2009 22:11:12 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Gregory Haskins <gregory.haskins@...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,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCHv7 3/3] vhost_net: a kernel-level virtio server

Michael S. Tsirkin a écrit :
> 
> 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?
> 

So you believe I am over-reacting to this dubious use of RCU ?

RCU documentation is already very complex, we dont need to add yet another
subtle use, and makes it less readable.

It seems you use 'RCU api' in drivers/vhost/net.c as convenient macros :

#define rcu_dereference(p)     ({ \
                                typeof(p) _________p1 = ACCESS_ONCE(p); \
                                smp_read_barrier_depends(); \
                                (_________p1); \
                                })

#define rcu_assign_pointer(p, v) \
        ({ \
                if (!__builtin_constant_p(v) || \
                    ((v) != NULL)) \
                        smp_wmb(); \
                (p) = (v); \
        })


There are plenty regular uses of smp_wmb() in kernel, not related to Read Copy Update,
there is nothing wrong to use barriers with appropriate comments.

(And you already use mb(), wmb(), rmb(), smp_wmb() in your patch)


BTW there is at least one locking bug in vhost_net_set_features()

Apparently, mutex_unlock() doesnt trigger a fault if mutex is not locked
by current thread... even with DEBUG_MUTEXES / DEBUG_LOCK_ALLOC


static void vhost_net_set_features(struct vhost_net *n, u64 features)
{
       size_t hdr_size = features & (1 << VHOST_NET_F_VIRTIO_NET_HDR) ?
               sizeof(struct virtio_net_hdr) : 0;
       int i;
<<!>>  mutex_unlock(&n->dev.mutex);
       n->dev.acked_features = features;
       smp_wmb();
       for (i = 0; i < VHOST_NET_VQ_MAX; ++i) {
               mutex_lock(&n->vqs[i].mutex);
               n->vqs[i].hdr_size = hdr_size;
               mutex_unlock(&n->vqs[i].mutex);
       }
       mutex_unlock(&n->dev.mutex);
       vhost_net_flush(n);
}
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