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.
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Notes:
svn2git
2020-12-20 02:59:44 +00:00
svn path=/head/; revision=280989
@ -254,6 +254,20 @@ ip_fillid(struct ip *ip)
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ip->ip_id = ip_randomid();
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else {
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counter_u64_add(V_ip_id, 1);
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/*
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* There are two issues about this trick, to be kept in mind.
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* 1) We can migrate between counter_u64_add() and next
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* line, and grab counter from other CPU, resulting in too
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* quick ID reuse. This is tolerable in our particular case,
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* since probability of such event is much lower then reuse
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* of ID due to legitimate overflow, that at modern Internet
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* speeds happens all the time.
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* 2) We are relying on the fact that counter(9) is based on
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* UMA_ZONE_PCPU uma(9) zone. We also take only last
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* sixteen bits of a counter, so we don't care about the
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* fact that machines with 32-bit word update their counters
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* not atomically.
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*/
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ip->ip_id = htons((*(uint64_t *)zpcpu_get(V_ip_id)) & 0xffff);
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}
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}
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