Date: Wed, 7 Mar 2012 17:51:52 -0800 From: prabhakar lakhera <prabhakar.lakhera@gmail.com> To: freebsd-net@freebsd.org Subject: Doubt regarding IPv6 DAD detection code Message-ID: <CALg%2BrhVkcisVSP39v18UkwP90MdLDRWGAS-ci5uCBb3BWZecog@mail.gmail.com>
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Hi,
I was puzzled to look at DAD detection code in FreeBSD. We check for
counters for any received NA/NS for DAD in nd6_dad_timer:
if (dp->dad_na_icount) {
1326 /*
1327 * the check is in nd6_dad_na_input(),
1328 * but just in case
1329 */
1330 duplicate++;
1331 }
1332
1333 if (dp->dad_ns_icount) {
1334 /* We've seen NS, means DAD has failed. */
1335 duplicate++;
1336 }
1337
1338 if (duplicate) {
1339 /* (*dp) will be freed in
nd6_dad_duplicated() */
1340 dp = NULL;
1341 nd6_dad_duplicated(ifa);
the function later calls nd6_dad_duplicated to perform the remaining work
if the address is detected duplicate.
nd6_dad_duplicated also gets called from nd6_dad_na_input
and nd6_dad_ns_input, both the functions are the only places which
increment the input NA/NS counters respectively.
1505 static void
1506 nd6_dad_na_input(struct ifaddr *ifa)
1507 {
1508 struct dadq *dp;
1509
1510 if (ifa == NULL)
1511 panic("ifa == NULL in nd6_dad_na_input");
1512
1513 dp = nd6_dad_find(ifa);
1514 if (dp)
1515 dp->dad_na_icount++;
1516
1517 /* remove the address. */
1518 nd6_dad_duplicated(ifa);
1519 }
nd6_dad_duplicated stops the timer among other things.
Why nd6_dad_timer need check on these counters if we stop the timer on DAD
failure anyways?
Ok.. may be just an optimization which just "hopes" that the counters have
been updated but the nd6_dad_*_input has not yet called nd6_dad_duplicated.
Can the this timer and na packet processing ever run in parallel, I don;t
see dp being protected by any locks, nor does it seem that it's been
reference counted.
Any explanation will be highly appreciated.
Best,
Prabhakar
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