Author: glebius
Date: Thu Apr 2 14:22:59 2015
New Revision: 280989
URL: https://svnweb.freebsd.org/changeset/base/280989
Log:
Provide a comment explaining issues with the counter(9) trick, so that
people won't copy and paste it blindly.
Prodded by: ian
Sponsored by: Nginx, Inc.
Modified:
head/sys/netinet/ip_id.c
Modified: head/sys/netinet/ip_id.c
==============================================================================
--- head/sys/netinet/ip_id.c Thu Apr 2 13:51:06 2015 (r280988)
+++ head/sys/netinet/ip_id.c Thu Apr 2 14:22:59 2015 (r280989)
@@ -254,6 +254,20 @@ ip_fillid(struct ip *ip)
ip->ip_id = ip_randomid();
else {
counter_u64_add(V_ip_id, 1);
+ /*
+ * There are two issues about this trick, to be kept in mind.
+ * 1) We can migrate between counter_u64_add() and next
+ * line, and grab counter from other CPU, resulting in too
+ * quick ID reuse. This is tolerable in our particular case,
+ * since probability of such event is much lower then reuse
+ * of ID due to legitimate overflow, that at modern Internet
+ * speeds happens all the time.
+ * 2) We are relying on the fact that counter(9) is based on
+ * UMA_ZONE_PCPU uma(9) zone. We also take only last
+ * sixteen bits of a counter, so we don't care about the
+ * fact that machines with 32-bit word update their counters
+ * not atomically.
+ */
ip->ip_id = htons((*(uint64_t *)zpcpu_get(V_ip_id)) & 0xffff);
}
}
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