Re: [PATCH] dmaengine:idxd: Use local64_try_cmpxchg in perfmon_pmu_event_update

[Tom Zanussi]

On Wed, 2023-07-05 at 07:53 -0700, Dave Jiang wrote:
> 
> 
> On 7/3/23 07:52, Uros Bizjak wrote:
> > Use local64_try_cmpxchg instead of local64_cmpxchg (*ptr, old, new) == old
> > in perfmon_pmu_event_update.  x86 CMPXCHG instruction returns success in
> > ZF flag, so this change saves a compare after cmpxchg (and related move
> > instruction in front of cmpxchg).
> > 
> > Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
> > fails. There is no need to re-read the value in the loop.
> > 
> > No functional change intended.
> > 
> > Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
> > Cc: Dave Jiang <dave.jiang@xxxxxxxxx>
> > Cc: Vinod Koul <vkoul@xxxxxxxxxx>
> > Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
> 
> 
> Cc: Tom Zanussi
> 
> Tom do you mind review this patch? Thanks!

Looks fine to me.

Thanks,

Reviewed-by: Tom Zanussi <tom.zanussi@xxxxxxxxxxxxxxx>

> 
> > ---
> >   drivers/dma/idxd/perfmon.c | 7 +++----
> >   1 file changed, 3 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/dma/idxd/perfmon.c b/drivers/dma/idxd/perfmon.c
> > index d73004f47cf4..fdda6d604262 100644
> > --- a/drivers/dma/idxd/perfmon.c
> > +++ b/drivers/dma/idxd/perfmon.c
> > @@ -245,12 +245,11 @@ static void perfmon_pmu_event_update(struct perf_event *event)
> >         int shift = 64 - idxd->idxd_pmu->counter_width;
> >         struct hw_perf_event *hwc = &event->hw;
> >   
> > +       prev_raw_count = local64_read(&hwc->prev_count);
> >         do {
> > -               prev_raw_count = local64_read(&hwc->prev_count);
> >                 new_raw_count = perfmon_pmu_read_counter(event);
> > -       } while (local64_cmpxchg(&hwc->prev_count, prev_raw_count,
> > -                       new_raw_count) != prev_raw_count);
> > -
> > +       } while (!local64_try_cmpxchg(&hwc->prev_count,
> > +                                     &prev_raw_count, new_raw_count));
> >         n = (new_raw_count << shift);
> >         p = (prev_raw_count << shift);
> >