From 7a742e374434e12eb6928b254ecde4b8f1cd809b Mon Sep 17 00:00:00 2001 From: Gleb Smirnoff Date: Thu, 2 Apr 2015 14:22:59 +0000 Subject: [PATCH] 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. --- sys/netinet/ip_id.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+) diff --git a/sys/netinet/ip_id.c b/sys/netinet/ip_id.c index 9df7148e46d4..b0efbee0b262 100644 --- a/sys/netinet/ip_id.c +++ b/sys/netinet/ip_id.c @@ -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); } }