On Sat, Nov 24, 2018 at 02:27:53PM +0000, Parav Pandit wrote:
> > > +static u64 rxe_stats_read_by_index(const struct rxe_dev *dev, int
> > > +index) {
> > > + struct rxe_stats *stats;
> > > + u64 sum = 0;
> > > + int cpu;
> > > +
> > > + for_each_possible_cpu(cpu) {
> > > + stats = per_cpu_ptr(dev->pcpu_stats, cpu);
> > > + sum += stats->counters[index];
> > > + }
> >
> > This is still racy with a writer on another CPU.
>
> I didn't follow you. If other cpu is updating counter for given
> index, sum will be updated to user in next read cycle whenever user
> does it. Counters are always a snapshot when read. Idea here is to
> have right sum when multiple cpu writers are writing, which was not
> the case before this patch.
Where is the snapshot?
Reading a u64 is not possible atomically on 32 bit platforms, you get
garbage if there is a race with a writer.
Jason
