Re: [Intel-gfx] [PATCH] drm/i915/perf: don't read head/tail pointers outside critical section

["Dixit, Ashutosh" <ashutosh.dixit@intel.com>]

On Mon, 30 Mar 2020 03:09:20 -0700, Chris Wilson wrote:
>
> Quoting Lionel Landwerlin (2020-03-30 10:14:11)
> > Reading or writing those fields should only happen under
> > stream->oa_buffer.ptr_lock.
>
> Writing, yes. Reading as a pair, sure. There are other ways you can
> ensure that the tail/head are read as one, but fair enough.

Sorry but I am trying to understand exactly what the purpose of
stream->oa_buffer.ptr_lock is? This is a classic ring buffer producer
consumer situation where producer updates tail and consumer updates
head. Since both are u32's can't those operations be done without requiring
a lock?

>
> > Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
> > Fixes: d1df41eb72ef ("drm/i915/perf: rework aging tail workaround")
> > ---
> >  drivers/gpu/drm/i915/i915_perf.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
> > index c74ebac50015..ec9421f02ebd 100644
> > --- a/drivers/gpu/drm/i915/i915_perf.c
> > +++ b/drivers/gpu/drm/i915/i915_perf.c
> > @@ -463,6 +463,7 @@ static bool oa_buffer_check_unlocked(struct i915_perf_stream *stream)
> >         u32 gtt_offset = i915_ggtt_offset(stream->oa_buffer.vma);
> >         int report_size = stream->oa_buffer.format_size;
> >         unsigned long flags;
> > +       bool pollin;
> >         u32 hw_tail;
> >         u64 now;
> >
> > @@ -532,10 +533,13 @@ static bool oa_buffer_check_unlocked(struct i915_perf_stream *stream)
> >                 stream->oa_buffer.aging_timestamp = now;
> >         }
> >
> > +       pollin = OA_TAKEN(stream->oa_buffer.tail - gtt_offset,
> > +                         stream->oa_buffer.head - gtt_offset) >= report_size;
> > +
> > +
>
> Bonus \n
>
> Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
>
> >         spin_unlock_irqrestore(&stream->oa_buffer.ptr_lock, flags);
> >
> > -       return OA_TAKEN(stream->oa_buffer.tail - gtt_offset,
> > -                       stream->oa_buffer.head - gtt_offset) >= report_size;
> > +       return pollin;

In what way is the original code incorrect? As I mentioned head is u32 and
can be read atomically without requiring a lock? We had deliberately moved
this code outside the lock so as to pick up the the latest value of head if
it had been updated in the consumer (read).
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