[PATCH 1/3] drm/i915: Hold a ref to the ring while retiring

[Chris Wilson]

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.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 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 ++++++++++++-
 5 files changed, 26 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);
+	}
 }
 
 #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 b0aa1f0d4e47..88ed7ba8886f 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>
 
@@ -51,6 +52,7 @@ 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 dc3de09c7586..00fa4a3bc9a3 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);
@@ -2869,7 +2869,7 @@ 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 f26f5cc1584c..ebac752e092d 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -1311,6 +1311,7 @@ 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);
 
@@ -1335,9 +1336,9 @@ 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);
@@ -1591,7 +1592,7 @@ 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;
@@ -1605,7 +1606,7 @@ 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);
-- 
2.20.1