Le mardi 31 août 2010 à 16:35 -0500, Christoph Lameter a écrit : > 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? > IMHO, this_cpu_read() is fine as is : a _read_ operation. Dont pretend it can be used in every context, its not true. -- 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>