Re: [PATCH 3/4]percpu_counter: fix code for 32bit systems

From: Shaohua Li
Date: Tue Apr 12 2011 - 21:05:43 EST


On Wed, 2011-04-13 at 03:02 +0800, Tejun Heo wrote:
> On Tue, Apr 12, 2011 at 04:04:04PM +0800, Shaohua Li wrote:
> > static inline s64 percpu_counter_read(struct percpu_counter *fbc)
> > {
> > +#if BITS_PER_LONG == 32
> > + s64 count;
> > + spin_lock(&fbc->lock);
> > + count = fbc->count;
> > + spin_unlock(&fbc->lock);
> > + return count;
> > +#else
> > return fbc->count;
> > +#endif
>
> I don't know. Is there any problem caused by this? The interface is
> known to be unreliable and already being used in speculative manner.
> I think it's more beneficial to avoid using locks on fast read path.
yes, it is unreliable, but only in an extent of batch*nr_cpus. accessing
64bits in 32bit machine can give us a _very_ big inaccuracy, which is
unacceptable to me.

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