The Go Programming Language

9.3 Read/Write Mutexes: sync.RWMutex

In a fit of anxiety after seeing his $100 deposit vanish without a trace, Bob writes a program to check his bank balance hundreds of times a second. He runs it at home, at work, and on his phone. The bank notices that the increased traffic is delaying deposits and withdrawals, because all the Balance requests run sequentially, holding the lock exclusively and temporarily preventing other goroutines from running.

Since the Balance function only needs to read the state of the variable, it would in fact be safe for multiple Balance calls to run concurrently, so long as no Deposit or Withdraw call is running. In this scenario we need a special kind of lock that allows read-only operations to proceed in parallel ...