The code is multi-threaded, and uses mutexes and the CRITICAL_BLOCK/TRY_CRITICAL_BLOCK macros to protect data structures.
Deadlocks due to inconsistent lock ordering (thread 1 locks cs_main and then cs_wallet, while thread 2 locks them in the opposite order: result, deadlock as each waits for the other to release its lock) are a problem. Compile with -DDEBUG_LOCKORDER to get lock order inconsistencies reported in the debug.log file.
Re-architecting the core code so there are better-defined interfaces between the various components is a goal, with any necessary locking done by the components (e.g. see the self-contained CKeyStore class and its cs_KeyStore lock for example).
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Threads
StartNode : Starts other threads.
ThreadGetMyExternalIP : Determines outside-the-firewall IP address, sends addr message to connected peers when it determines it.
ThreadIRCSeed : Joins IRC bootstrapping channel, watching for new peers and advertising this node's IP address.
ThreadSocketHandler : Sends/Receives data from peers on port 8333.
ThreadMessageHandler : Higher-level message handling (sending and receiving).
ThreadOpenConnections : Initiates new connections to peers.
ThreadTopUpKeyPool : replenishes the keystore's keypool.
ThreadCleanWalletPassphrase : re-locks an encrypted wallet after user has unlocked it for a period of time.
SendingDialogStartTransfer : used by pay-via-ip-address code (obsolete)
ThreadDelayedRepaint : repaint the gui
ThreadFlushWalletDB : Close the wallet.dat file if it hasn't been used in 500ms.
ThreadRPCServer : Remote procedure call handler, listens on port 8332 for connections and services them.