Re: [RFC PATCH -v2] percpu_counters: make fbc->count read atomic on 32 bit architecture

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Mon, 2008-08-25 at 19:35 +0530, Aneesh Kumar K.V wrote:
> On Mon, Aug 25, 2008 at 01:27:19PM +0200, Peter Zijlstra wrote:
> > On Mon, 2008-08-25 at 16:50 +0530, Aneesh Kumar K.V wrote:

> > > @@ -53,10 +53,31 @@ static inline s64 percpu_counter_sum(struct percpu_counter *fbc)
> > >  	return __percpu_counter_sum(fbc);
> > >  }
> > >  
> > > -static inline s64 percpu_counter_read(struct percpu_counter *fbc)
> > > +#if BITS_PER_LONG == 64
> > > +static inline s64 fbc_count(struct percpu_counter *fbc)
> > >  {
> > >  	return fbc->count;
> > >  }
> > > +#else
> > > +/* doesn't have atomic 64 bit operation */
> > > +static inline s64 fbc_count(struct percpu_counter *fbc)
> > > +{
> > > +	s64 ret;
> > > +	unsigned seq;
> > > +	unsigned long flags;
> > > +	do {
> > > +		seq = read_seqbegin_irqsave(&fbc->lock, flags);
> > > +		ret = fbc->count;
> > > +	} while(read_seqretry_irqrestore(&fbc->lock, seq, flags));
> > 
> > Do we really need to disabled IRQs here? It seems to me the worst that
> > can happen is that the IRQ will change ->count and increase the sequence
> > number a bit - a case that is perfectly handled by the current retry
> > logic.
> > 
> > And not doing the IRQ flags bit saves a lot of time on some archs.
> > 
> 
> Will update in the next version. BTW does it make sense to do
> the above unconditionally now ? ie to remove the #if ?. How much
> impact would it be to do read_seqbegin and read_seqretry on a 64bit
> machine too ?

there's a few smp_rmb()s in there - so that will at the very least be a
compiler barrier and thus generate slightly worse code along with the
few extra reads.

But I'm not sure that's measurable..