Quoting Tvrtko Ursulin (2020-07-17 13:38:01)
>
> On 15/07/2020 12:50, Chris Wilson wrote:
> > + /* Even if we have not used the cache, we may still have a barrier */
> > + if (!ref->cache)
> > + ref->cache = fetch_node(ref->tree.rb_node);
> > +
> > + /* Keep the MRU cached node for reuse */
> > + if (ref->cache) {
> > + /* Discard all other nodes in the tree */
> > + rb_erase(&ref->cache->node, &ref->tree);
> > + root = ref->tree;
> > +
> > + /* Rebuild the tree with only the cached node */
> > + rb_link_node(&ref->cache->node, NULL, &ref->tree.rb_node);
> > + rb_insert_color(&ref->cache->node, &ref->tree);
> > + GEM_BUG_ON(ref->tree.rb_node != &ref->cache->node);
> > + }
> >
> > spin_unlock_irqrestore(&ref->tree_lock, flags);
> >
> > @@ -156,6 +168,7 @@ __active_retire(struct i915_active *ref)
> > /* ... except if you wait on it, you must manage your own references! */
> > wake_up_var(ref);
> >
> > + /* Finally free the discarded timeline tree */
> > rbtree_postorder_for_each_entry_safe(it, n, &root, node) {
> > GEM_BUG_ON(i915_active_fence_isset(&it->base));
> > kmem_cache_free(global.slab_cache, it);
>
> Here it frees everything.. so how does ref->cache, being in the tree,
> survives?
This is the old root which does not contain ref->cache, as we moved that
to become the new tree.
/* Discard all other nodes in the tree */
rb_erase(&ref->cache->node, &ref->tree);
root = ref->tree;
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