LoadChainTip requires cs_main to be locked. Under some conditions, it may call ActivateBestChain. However, ActivateBestChain requires cs_main NOT to be taken when called.
{
    if (!fCheckBlockIndex) {
        return;
    }
    LOCK(cs_main);
    
    
    
    if (m_chain.Height() < 0) {
        assert(mapBlockIndex.size() <= 1);
        return;
    }
    
    std::multimap<CBlockIndex*,CBlockIndex*> forward;
    for (const std::pair<const uint256, CBlockIndex*>& entry : mapBlockIndex) {
        forward.insert(std::make_pair(entry.second->pprev, entry.second));
    }
    assert(forward.size() == mapBlockIndex.size());
    std::pair<std::multimap<CBlockIndex*,CBlockIndex*>::iterator,std::multimap<CBlockIndex*,CBlockIndex*>::iterator> rangeGenesis = forward.equal_range(nullptr);
    CBlockIndex *pindex = rangeGenesis.first->second;
    rangeGenesis.first++;
    assert(rangeGenesis.first == rangeGenesis.second);