Re: [PATCH 2/7] CAN: Add PF_CAN core module

Patrick McHardy
2007-09-19T08:34:54Z

Urs Thuermann wrote:
> Patrick McHardy <kaber@trash.net> writes:
>
>>>+HLIST_HEAD(rx_dev_list);
>>
>>
>>Same here (static).
>
>
> Can't be static since it's used in proc.c. But __read_mostly might
> make sense.
>
> What exactly is the effect of __read_mostly? Is that in a separate
> ELF section? Where is that finally located?

Its a seperate section to prevent false sharing.

>
>
>>>+ if (ret == -ENOSYS)
>>>+ printk(KERN_INFO "can: request_module(%s) not"
>>>+ " implemented.\n", module_name);
>>>+ else if (ret)
>>>+ printk(KERN_ERR "can: request_module(%s) failed\n",
>>>+ module_name);
>>
>>
>>Both of these printks seem to be user-triggerable, so they should
>>be rate-limited (or maybe get removed completely/changed to DBG).
>
>
> Hm, I don't think DBG() would be right here, since the messages show
> problems in the installation to the admin. OTOH, I see that a user
> can flood the log by opening sockets with invalid proto numbers. Rate
> limiting might solve this, or we should print the message only once
> per proto number. I will think about this.

IMO this is a perfectly normal condition (not finding a module).
Especially the !KMOD case is hardly an error.

>>>+ /* check for success and correct type */
>>>+ cp = proto_tab[protocol];
>>
>>
>>What prevents the module from getting unloaded again (and using
>>a stale pointer)?
>
>
> When the module is unloaded it calls can_proto_unregister() which
> clears the pointer. Do you see a race condition here?

Yes, you do request_module, load the module, get the cp pointer
from proto_tab, the module is unloaded again. cp points to
stable memory. Using module references would fix this.

>>>+ if (!(skb->dev->flags & IFF_LOOPBACK)) {
>>>+ /*
>>>+ * If the interface is not capable to do loopback
>>>+ * itself, we do it here.
>>>+ */
>>>+ struct sk_buff *newskb = skb_clone(skb, GFP_ATOMIC);
>>>+
>>>+ if (!newskb) {
>>>+ kfree_skb(skb);
>>>+ return -ENOMEM;
>>>+ }
>>>+
>>>+ newskb->sk = skb->sk;
>>>+ newskb->ip_summed = CHECKSUM_UNNECESSARY;
>>>+ newskb->pkt_type = PACKET_BROADCAST;
>>>+ netif_rx(newskb);
>>
>>
>>So the intention here is to send the packet to the non-loopback device
>>and manually loop it, which means sending it twice?
>
>
> CAN is a broadcast message network, so every frame should be (usually)
> sent to all receivers, on remote hosts and to all local sockets. If
> the driver for the interface is not able to loop back the frame for
> local delivery, the can_send() function will do this as a fallback.
> For real CAN devices it is preferred that the driver does loopback.
> For vcan it makes no difference where loopback is done. The module
> paramenter for vcan is therefore only useful to test and debug the CAN
> core module. It is nothing a normal user will ever use.

Thanks for the explanation.

>>>+static struct dev_rcv_lists *find_dev_rcv_lists(struct net_device *dev)
>>>+{
>>>+ struct dev_rcv_lists *d;
>>>+ struct hlist_node *n;
>>>+
>>>+ ....
>>>+
>>>+ hlist_for_each_entry(d, n, &rx_dev_list, list) {
>>
>>
>>On the receive path you use RCU, so this should be
>>hlist_for_each_entry_rcu(), no? The bottem half disabling during
>>addition/removal also seems unnecessary, but I might be missing
>>something.
>
>
> find_dev_rcv_lists() is called in one place from can_rcv() with RCU
> lock held, as you write. The other two calls to find_dev_rcv_lists()
> are from can_rx_register/unregister() functions which change the
> receive lists. Therefore, we can't only use RCU but need protection
> against simultanous writes. We do this with the spin_lock_bh(). The
> _bh variant, because can_rcv() runs in interrupt and we need to block
> that. I thought this is pretty standard.
>
> I'll check this again tomorrow, but I have put much time in these
> locking issues already, changed it quite a few times and hoped to have
> got it right finally.

I'm not saying you should use *only* RCU, you need the lock
for additions/removal of course, but since the receive path
doesn't take that lock and relies on RCU, you need to use
the _rcu list walking variant to avoid races with concurrent
list changes.

>>>+ case NETDEV_REGISTER:
>>>+
>>>+ /*
>>>+ * create new dev_rcv_lists for this device
>>>+ *
>>>+ * N.B. zeroing the struct is the correct initialization
>>>+ * for the embedded hlist_head structs.
>>>+ * Another list type, e.g. list_head, would require
>>>+ * explicit initialization.
>>>+ */
>>>+
>>>+ DBG("creating new dev_rcv_lists for %s\n", dev->name);
>>>+
>>>+ d = kzalloc(sizeof(*d),
>>>+ in_interrupt() ? GFP_ATOMIC : GFP_KERNEL);
>>
>>
>>netdevice registration should never happen from interrupt handlers.
>
>
> Hm, I seem to remember we had such an occurance with hot-pluggable
> devices, i.e. USB. But I may be wrong. In what context is
> NETDEV_REGISTER called for e.g. USB devices? Can we safely write
> just GFP_KERNEL here?

Yes. register_netdevice() takes the rtnl mutex, so it can only
be used in process context.

>>>+ stattimer.expires = jiffies + HZ;
>>
>>round_jiffies?
>
>
> Yes. We don't depend on exact times relative to module load time, but
> we would like to have the timer expirations 1 second apart. But we
> would still get this with round_jiffies(), right?

I don't think you will get *exactly* one second, but you also have
no guarantee for that now.

>>>+ * proc read functions
>>>+ *
>>>+ * From known use-cases we expect about 10 entries in a receive list to be
>>>+ * printed in the proc_fs. So PAGE_SIZE is definitely enough space here.
>>
>>
>>Would be nicer to use seq_file (for all the proc stuff).
>
>
> That has already been on my TODO list. There was some problem which I
> don't remember ATM, which is why I dropped it temporarily. Now on my
> TODO list again.

Thanks :)