`RewindBlockIndex` reads `chainActive` (via `ChainActive()`) without holding `cs_main`:

https://github.com/bitcoin/bitcoin/blob/b2a6b0216192b6e8428f1a80b47f5178fccb961a/src/validation.cpp#L4314-L4331

(Please note that callers of `RewindBlockIndex` are _not_ allowed to hold `cs_main` since `RewindBlockIndex` indirectly calls `LimitValidationInterfaceQueue`.)
{
    bool fClean = true;
    if (view.HaveCoin(out)) fClean = false; 
    if (undo.nHeight == 0) {
        
        
        
        const Coin& alternate = AccessByTxid(view, out.hash);
        if (!alternate.IsSpent()) {
            undo.nHeight = alternate.nHeight;
            undo.fCoinBase = alternate.fCoinBase;
        } else {
            return DISCONNECT_FAILED; 
        }
    }
    
    
    
    
    view.AddCoin(out, std::move(undo), !fClean);
    return fClean ? DISCONNECT_OK : DISCONNECT_UNCLEAN;
}
/** Undo the effects of this block (with given index) on the UTXO set represented by coins.
 *  When FAILED is returned, view is left in an indeterminate state. */
DisconnectResult CChainState::DisconnectBlock(const CBlock& block, const CBlockIndex* pindex, CCoinsViewCache& view)