Re: [PATCH 03/10] Use percpu stats

Eric Dumazet
2010-08-31T21:41:24Z

Le mardi 31 aoA>>t 2010 A 16:35 -0500, Christoph Lameter a A(C)crit :
> 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.