[PATCH 2/9] drm/i915: Check waiter->seqno carefully in case of preemption

[Chris Wilson]

If preemption occurs at precisely the right moment, we may decide that
the wait is complete even though the wait's request is no longer
executing (having been preempted). We handle this situation by double
checking that request following deciding whether the wait is complete.

Reported-by: Michał Winiarski <michal.winiarski at intel.com
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Michał Winiarski <michal.winiarski at intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski at intel.com>
---
 drivers/gpu/drm/i915/i915_irq.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index 2190e42ba310..bd38c6983eec 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -1053,10 +1053,13 @@ static void notify_ring(struct intel_engine_cs *engine)
 		 */
 		if (i915_seqno_passed(intel_engine_get_seqno(engine),
 				      wait->seqno)) {
+			struct drm_i915_gem_request *waiter = wait->request;
+
 			wakeup = true;
 			if (!test_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
-				      &wait->request->fence.flags))
-				rq = i915_gem_request_get(wait->request);
+				      &waiter->fence.flags) &&
+			    intel_wait_check_request(wait, waiter))
+				rq = i915_gem_request_get(waiter);
 		}
 
 		if (wakeup)
-- 
2.14.1