On Tue, 31 Aug 2010, Eric Dumazet wrote: > > Yes, this_cpu_add() seems sufficient. I can't recall why I used u64_stats_* > > but if it's not required for atomic access to 64-bit then why was it added to > > the mainline in the first place? > > Because we wanted to have fast 64bit counters, even on 32bit arches, and > this has litle to do with 'atomic' on one entity, but a group of > counters. (check drivers/net/loopback.c, lines 91-94). No lock prefix > used in fast path. > > We also wanted readers to read correct values, not a value being changed > by a writer, with inconsistent 32bit halves. SNMP applications want > monotonically increasing counters. > > this_cpu_add()/this_cpu_read() doesnt fit. > > Even for single counter, this_cpu_read(64bit) is not using an RMW > (cmpxchg8) instruction, so you can get very strange results when low > order 32bit wraps. How about fixing it so that everyone benefits? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>