drm/i915/gt: Clear the buffer pool age before use

If we create a new node, it is possible for the slab allocator to return
us a recently freed node. If that node was just retired, it will retain
the current jiffy as its node->age. There is then a miniscule window,
where as that node is retired, it will appear on the free list with an
incorrect age and be eligible for reuse by one thread, and then by a
second thread as the correct node->age is written.

Fixes: 06b73c2d0b ("drm/i915/gt: Delay taking the spinlock for grabbing from the buffer pool")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200915091417.4086-3-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_gt_buffer_pool.c

@@ -134,6 +134,7 @@ static void pool_retire(struct i915_active *ref)
 	/* Return this object to the shrinker pool */
 	i915_gem_object_make_purgeable(node->obj);
 
+	GEM_BUG_ON(node->age);
 	spin_lock_irqsave(&pool->lock, flags);
 	list_add_rcu(&node->link, list);
 	WRITE_ONCE(node->age, jiffies ?: 1); /* 0 reserved for active nodes */
@@ -155,6 +156,7 @@ node_create(struct intel_gt_buffer_pool *pool, size_t sz)
 	if (!node)
 		return ERR_PTR(-ENOMEM);
 
+	node->age = 0;
 	node->pool = pool;
 	i915_active_init(&node->active, pool_active, pool_retire);