[lwip-users] "the ARP layer is not protected against concurrent access"

[Goldschmidt Simon]

Hi list,

CHANGELOG says:
FUTURE
...
  * TODO: the ARP layer is not protected against concurrent access. If
    you run from a multitasking OS, serialize access to ARP (called from
    your network device driver and from a timeout thread.)

My idea was to do all ARP processing in context of the tcpip_thread (like Frédéric suggested). In fact, I have already done so in my local sources (and of course, it works for a while now :)

I've done this by adding 2 new functions (and some new messages) to tcpip.h/.c:

err_t tcpip_input_w_arp(struct pbuf *p, struct netif *inp);
  for incoming IP packets, ETH header not removed to update ARP entry
err_t tcpip_arp_input(struct pbuf *p, struct netif *inp);
  for incoming ARP packets

(tcpip_input() would not be needed any more, but you have the choice when writing the ethernetif.c)

All access to etharp.c is thus done in the context of the tcpip_thread, which saves us the concurrent access protection. Downside is that ARP requests take a little longer to process (context switch) but I think that's a minor one.

AS far as I know (but I'm not using it by now), that wouldn't affect raw API programs running without an OS, since they don't use messages, anyway. This would simply be a matter of assigning another input function for netifs (and maybe another input function, e.g. netif->arp_input()).

Please send your comments, or I will check in my changes and you'll have to live with it ;-)

Simon