[Intel-gfx] [PATCH] drm/i915/gt: Pull release of node->age under the spinlock

[Chris Wilson]

We need to ensure that the list is valid prior to marking the node as
retrievable, otherwise we may see two threads compete over the same node
in intel_gt_get_buffer_pool(). If the first thread acquires and releases
the node in the same jiffie, the second thread may then acquire it (as
the jiffie now again matches the expected value) and claim the node
before it is put back into the list.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/gt/intel_gt_buffer_pool.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_gt_buffer_pool.c b/drivers/gpu/drm/i915/gt/intel_gt_buffer_pool.c
index 9e938d4f6bfe..4b7671ac5dca 100644
--- a/drivers/gpu/drm/i915/gt/intel_gt_buffer_pool.c
+++ b/drivers/gpu/drm/i915/gt/intel_gt_buffer_pool.c
@@ -134,9 +134,9 @@ static void pool_retire(struct i915_active *ref)
 	/* Return this object to the shrinker pool */
 	i915_gem_object_make_purgeable(node->obj);
 
-	WRITE_ONCE(node->age, jiffies ?: 1); /* 0 reserved for active nodes */
 	spin_lock_irqsave(&pool->lock, flags);
 	list_add_rcu(&node->link, list);
+	WRITE_ONCE(node->age, jiffies ?: 1); /* 0 reserved for active nodes */
 	spin_unlock_irqrestore(&pool->lock, flags);
 
 	schedule_delayed_work(&pool->work,
-- 
2.20.1