[PATCH] drm/i915: Ensure associated VMAs are inactive when contexts are destroyed

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

In the following commit:

    commit e9f24d5fb7cf3628b195b18ff3ac4e37937ceeae
    Author: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
    Date:   Mon Oct 5 13:26:36 2015 +0100

        drm/i915: Clean up associated VMAs on context destruction

I added a WARN_ON assertion that VM's active list must be empty
at the time of owning context is getting freed, but that turned
out to be a wrong assumption.

Due ordering of operations in i915_gem_object_retire__read, where
contexts are unreferenced before VMAs are moved to the inactive
list, the described situation can in fact happen.

It feels wrong to do things in such order so this fix makes sure
a reference to context is held until the move to inactive list
is completed.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92638
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_gem.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 9b2048c7077d..6cbe3fdbca96 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2373,19 +2373,27 @@ static void
 i915_gem_object_retire__read(struct drm_i915_gem_object *obj, int ring)
 {
 	struct i915_vma *vma;
+	struct intel_context *ctx;
 
 	RQ_BUG_ON(obj->last_read_req[ring] == NULL);
 	RQ_BUG_ON(!(obj->active & (1 << ring)));
 
 	list_del_init(&obj->ring_list[ring]);
+
+	/* Ensure context cannot be destroyed with VMAs on the active list. */
+	ctx = obj->last_read_req[ring]->ctx;
+	i915_gem_context_reference(ctx);
+
 	i915_gem_request_assign(&obj->last_read_req[ring], NULL);
 
 	if (obj->last_write_req && obj->last_write_req->ring->id == ring)
 		i915_gem_object_retire__write(obj);
 
 	obj->active &= ~(1 << ring);
-	if (obj->active)
+	if (obj->active) {
+		i915_gem_context_unreference(ctx);
 		return;
+	}
 
 	/* Bump our place on the bound list to keep it roughly in LRU order
 	 * so that we don't steal from recently used but inactive objects
@@ -2399,6 +2407,8 @@ i915_gem_object_retire__read(struct drm_i915_gem_object *obj, int ring)
 			list_move_tail(&vma->mm_list, &vma->vm->inactive_list);
 	}
 
+	i915_gem_context_unreference(ctx);
+
 	i915_gem_request_assign(&obj->last_fenced_req, NULL);
 	drm_gem_object_unreference(&obj->base);
 }
-- 
1.9.1

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