NOTE
2.9 sync.RWMutex
Go's reader/writer mutex: concurrent readers, exclusive writers, writer preference, non-upgradable locking, and when RWMutex is actually useful.
This is a historical learning note and may contain outdated or incomplete understanding.
1. What Is sync.RWMutex?
RWMutex allows either:
- multiple concurrent readers holding
RLock; or - one exclusive writer holding
Lock.
mu.RLock()
v := shared[key]
mu.RUnlock()
mu.Lock()
shared[key] = value
mu.Unlock()Its zero value is ready to use, and it must not be copied after first use.
2. Writer Progress
When a writer is waiting, new readers are not allowed to indefinitely keep extending the reader set. This prevents continuous readers from starving a writer.
For this reason an RLock should not be held while waiting to acquire another RLock through a path that depends on a blocked writer progressing.
3. No Upgrade or Downgrade Primitive
RWMutex does not provide an atomic upgrade from RLock to Lock, nor an atomic downgrade from Lock to RLock. Releasing one mode and acquiring the other creates an interval in which another goroutine can modify the state.
4. When to Use It
RWMutex helps when:
- reads dominate;
- read critical sections are significant enough for concurrent reads to matter;
- write contention is relatively low.
For tiny critical sections, a plain Mutex may be simpler and equally fast or faster. Measure before introducing reader/writer complexity.