spinlock
rwlock_t spinlock;
read_lock_bh(&tree->u.spinlock);
read_unlock_bh(&tree->u.spinlock);
write_lock_bh(&tree->u.spinlock);
write_unlock_bh(&tree->u.spinlock);
write_lock_bh(&lh->cache->trees[index].u.spinlock);
read_lock_bh(&lh->cache->trees[index].u.spinlock);
write_unlock_bh(&lh->cache->trees[index].u.spinlock);
read_unlock_bh(&lh->cache->trees[index].u.spinlock);
struct spinlock {