[RFC] drm/i915: reference count batch object on requests

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

We used to lean on active_list to handle the references
to batch objects. But there are useful cases when same,
albeit simple, batch can be executing on multiple rings
concurrently. For this case the active_list reference count
handling is just not enough as batch could be freed by
ring A request retirement as it is still running on ring B.

Fix this by doing proper batch_obj reference counting.

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>

Notes:
    This is a patch which ameliorates the
    [PATCH] tests/gem_reset_stats: add close-pending-fork
    
    Chris wasn't happy about the refcounting as it might hide
    the true problem. But I haven't been able to find the real culprit,
    thus the RFC.
---
 drivers/gpu/drm/i915/i915_gem.c |   12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 40d9dcf..858538f 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2145,13 +2145,12 @@ int __i915_add_request(struct intel_ring_buffer *ring,
 	request->head = request_start;
 	request->tail = request_ring_position;
 
-	/* Whilst this request exists, batch_obj will be on the
-	 * active_list, and so will hold the active reference. Only when this
-	 * request is retired will the the batch_obj be moved onto the
-	 * inactive_list and lose its active reference. Hence we do not need
-	 * to explicitly hold another reference here.
+	/* Active list has one reference but that is not enough as same
+	 * batch_obj can be active on multiple rings
 	 */
 	request->batch_obj = obj;
+	if (request->batch_obj)
+		drm_gem_object_reference(&request->batch_obj->base);
 
 	/* Hold a reference to the current context so that we can inspect
 	 * it later in case a hangcheck error event fires.
@@ -2340,6 +2339,9 @@ static void i915_gem_free_request(struct drm_i915_gem_request *request)
 	if (request->ctx)
 		i915_gem_context_unreference(request->ctx);
 
+	if (request->batch_obj)
+		drm_gem_object_unreference(&request->batch_obj->base);
+
 	kfree(request);
 }
 
-- 
1.7.9.5