btrfs: track refs in a rb_tree instead of a list
If we get a significant amount of delayed refs for a single block (think modifying multiple snapshots) we can end up spending an ungodly amount of time looping through all of the entries trying to see if they can be merged. This is because we only add them to a list, so we have O(2n) for every ref head. This doesn't make any sense as we likely have refs for different roots, and so they cannot be merged. Tracking in a tree will allow us to break as soon as we hit an entry that doesn't match, making our worst case O(n). With this we can also merge entries more easily. Before we had to hope that matching refs were on the ends of our list, but with the tree we can search down to exact matches and merge them at insert time. Signed-off-by:Josef Bacik <jbacik@fb.com> Signed-off-by:
David Sterba <dsterba@suse.com>
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