doc: Add comment to cs_main and mempool::cs

src/validation.cpp

@@ -210,9 +210,17 @@ private:
     bool RollforwardBlock(const CBlockIndex* pindex, CCoinsViewCache& inputs, const CChainParams& params) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 } g_chainstate;
 
-
-
-CCriticalSection cs_main;
+/**
+ * Mutex to guard access to validation specific variables, such as reading
+ * or changing the chainstate.
+ *
+ * This may also need to be locked when updating the transaction pool, e.g. on
+ * AcceptToMemoryPool. See CTxMemPool::cs comment for details.
+ *
+ * The transaction pool has a separate lock to allow reading from it and the
+ * chainstate at the same time.
+ */
+RecursiveMutex cs_main;
 
 BlockMap& mapBlockIndex = g_chainstate.mapBlockIndex;
 CChain& chainActive = g_chainstate.chainActive;