drm/i915/perf: don't read head/tail pointers outside critical section

Reading or writing those fields should only happen under
stream->oa_buffer.ptr_lock.
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: d1df41eb ("drm/i915/perf: rework aging tail workaround")
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200330091411.37357-1-lionel.g.landwerlin@intel.com

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 +533,12 @@ 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;
+
 	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;
 }
 
 /**