Re: [PATCH 04/22] drm/i915: Hold a ref to the ring while retiring

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On 18/03/2019 09:51, Chris Wilson wrote:
As the final request on a ring may hold the reference to this ring (via
retiring the last pinned context), we may find ourselves chasing a
dangling pointer on completion of the list.

A quick solution is to hold a reference to the ring itself as we retire
along it so that we only free it after we stop dereferencing it.

Is there a guilty commit to reference as Fixes: ?


Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
---
  drivers/gpu/drm/i915/i915_request.c          |  6 +++++-
  drivers/gpu/drm/i915/intel_engine_types.h    |  2 ++
  drivers/gpu/drm/i915/intel_lrc.c             |  4 ++--
  drivers/gpu/drm/i915/intel_ringbuffer.c      |  9 +++++----
  drivers/gpu/drm/i915/intel_ringbuffer.h      | 13 ++++++++++++-
  drivers/gpu/drm/i915/selftests/mock_engine.c |  1 +
  6 files changed, 27 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index 9533a85cb0b3..0a3d94517d0a 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -1332,8 +1332,12 @@ void i915_retire_requests(struct drm_i915_private *i915)
  	if (!i915->gt.active_requests)
  		return;
- list_for_each_entry_safe(ring, tmp, &i915->gt.active_rings, active_link)
+	list_for_each_entry_safe(ring, tmp,
+				 &i915->gt.active_rings, active_link) {
+		intel_ring_get(ring); /* last rq holds reference! */
  		ring_retire_requests(ring);
+		intel_ring_put(ring);
+	}

Where does it chase a dangling pointer? It used the safe iterator already.

  }
#if IS_ENABLED(CONFIG_DRM_I915_SELFTEST)
diff --git a/drivers/gpu/drm/i915/intel_engine_types.h b/drivers/gpu/drm/i915/intel_engine_types.h
index 79a166b9a81b..549fdfca17aa 100644
--- a/drivers/gpu/drm/i915/intel_engine_types.h
+++ b/drivers/gpu/drm/i915/intel_engine_types.h
@@ -9,6 +9,7 @@
#include <linux/hashtable.h>
  #include <linux/irq_work.h>
+#include <linux/kref.h>
  #include <linux/list.h>
  #include <linux/types.h>
@@ -58,6 +59,7 @@ struct intel_engine_hangcheck {
  };
struct intel_ring {
+	struct kref ref;
  	struct i915_vma *vma;
  	void *vaddr;
diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index aa50f03ba812..d3f1fe06d013 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -1236,7 +1236,7 @@ static void execlists_submit_request(struct i915_request *request)
static void __execlists_context_fini(struct intel_context *ce)
  {
-	intel_ring_free(ce->ring);
+	intel_ring_put(ce->ring);
GEM_BUG_ON(i915_gem_object_is_active(ce->state->obj));
  	i915_gem_object_put(ce->state->obj);
@@ -2867,7 +2867,7 @@ static int execlists_context_deferred_alloc(struct intel_context *ce,
  	return 0;
error_ring_free:
-	intel_ring_free(ring);
+	intel_ring_put(ring);
  error_deref_obj:
  	i915_gem_object_put(ctx_obj);
  	return ret;
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 746fe570466c..45a54fadc482 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -1302,6 +1302,7 @@ intel_engine_create_ring(struct intel_engine_cs *engine,
  	if (!ring)
  		return ERR_PTR(-ENOMEM);
+ kref_init(&ring->ref);
  	INIT_LIST_HEAD(&ring->request_list);
  	ring->timeline = i915_timeline_get(timeline);
@@ -1326,9 +1327,9 @@ intel_engine_create_ring(struct intel_engine_cs *engine,
  	return ring;
  }
-void
-intel_ring_free(struct intel_ring *ring)
+void intel_ring_free(struct kref *ref)
  {
+	struct intel_ring *ring = container_of(ref, typeof(*ring), ref);
  	struct drm_i915_gem_object *obj = ring->vma->obj;
i915_vma_close(ring->vma);
@@ -1571,7 +1572,7 @@ static int intel_init_ring_buffer(struct intel_engine_cs *engine)
  err_unpin:
  	intel_ring_unpin(ring);
  err_ring:
-	intel_ring_free(ring);
+	intel_ring_put(ring);
  err:
  	intel_engine_cleanup_common(engine);
  	return err;
@@ -1585,7 +1586,7 @@ void intel_engine_cleanup(struct intel_engine_cs *engine)
  		(I915_READ_MODE(engine) & MODE_IDLE) == 0);
intel_ring_unpin(engine->buffer);
-	intel_ring_free(engine->buffer);
+	intel_ring_put(engine->buffer);
if (engine->cleanup)
  		engine->cleanup(engine);
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index e612bdca9fd9..a57489fcb302 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -231,7 +231,18 @@ int intel_ring_pin(struct intel_ring *ring);
  void intel_ring_reset(struct intel_ring *ring, u32 tail);
  unsigned int intel_ring_update_space(struct intel_ring *ring);
  void intel_ring_unpin(struct intel_ring *ring);
-void intel_ring_free(struct intel_ring *ring);
+void intel_ring_free(struct kref *ref);
+
+static inline struct intel_ring *intel_ring_get(struct intel_ring *ring)
+{
+	kref_get(&ring->ref);
+	return ring;
+}
+
+static inline void intel_ring_put(struct intel_ring *ring)
+{
+	kref_put(&ring->ref, intel_ring_free);
+}
void intel_engine_stop(struct intel_engine_cs *engine);
  void intel_engine_cleanup(struct intel_engine_cs *engine);
diff --git a/drivers/gpu/drm/i915/selftests/mock_engine.c b/drivers/gpu/drm/i915/selftests/mock_engine.c
index f6d120e05ee4..881450c694e9 100644
--- a/drivers/gpu/drm/i915/selftests/mock_engine.c
+++ b/drivers/gpu/drm/i915/selftests/mock_engine.c
@@ -57,6 +57,7 @@ static struct intel_ring *mock_ring(struct intel_engine_cs *engine)
  		return NULL;
  	}
+ kref_init(&ring->base.ref);
  	ring->base.size = sz;
  	ring->base.effective_size = sz;
  	ring->base.vaddr = (void *)(ring + 1);


Regards,

Tvrtko
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