Re: [patch v2 0/5] percpu_counter: bug fix and enhancement

[Shaohua Li]

Hi,
On Fri, May 13, 2011 at 01:20:06PM +0800, Eric Dumazet wrote:
> Le vendredi 13 mai 2011 à 12:37 +0800, Shaohua Li a écrit :
> > On Thu, 2011-05-12 at 17:05 +0800, Tejun Heo wrote:
> > > Hello,
> > > 
> > > On Thu, May 12, 2011 at 11:02:15AM +0200, Eric Dumazet wrote:
> > > > > I don't think @maxfuzzy is necessary there.  I wrote this before but
> > > > > why can't we track the actual deviation instead of the number of
> > > > > deviation events?
> > > > 
> > > > Thats roughly same thing (BATCH multiplicator factor apart)
> > > > 
> > > > Most percpu_counter users for a given percpu_counter object use a given
> > > > BATCH, dont they ?
> > > 
> > > Well, @maxfuzzy is much harder than @batch.  It's way less intuitive.
> > > Although I haven't really thought about it that much, I think it might
> > > be possible to eliminate it.  Maybe I'm confused.  I'll take another
> > > look later but if someone can think of something, please jump right
> > > in.
> > Hmm, looks Eric's approach doesn't work. because we want to remove lock
> > in _add, checking seq in _sum still races with _add.
> > 
> 
> Why ?
> 
> I'll code a patch, I believe it should work.
I thought your proposal is:
in _add
{
	if (count >= batch || count <= -batch) {
		fbc->seq_count++;
               atomic64_add(count, &fbc->count);
-------->
                __this_cpu_write(*fbc->counters, 0);
	}
}

in _sum
{
restart:
	oldseq = fbc->seqcount;
	smp_rmb();
	do_sum();
	smp_rmb()
	newseq = fbc->seqcount;
	if (newseq - oldseq >= maxfuzzy)
		goto restart;
	return ret;
} 
if _sum run between above line marked in _add, then the seqcount check
doesn't work, we still have deviation Tejun pointed out.

Thanks,
Shaohua
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