src/sync/rwmutex.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package sync

import (
	"internal/race"
	"sync/atomic"
	"unsafe"
)

// An RWMutex is a reader/writer mutual exclusion lock.
// The lock can be held by an arbitrary number of readers or a single writer.
// RWMutexes can be created as part of other structures;
// the zero value for a RWMutex is an unlocked mutex.
//
// An RWMutex must not be copied after first use.
//
// If a goroutine holds a RWMutex for reading, it must not expect this or any
// other goroutine to be able to also take the read lock until the first read
// lock is released. In particular, this prohibits recursive read locking.
// This is to ensure that the lock eventually becomes available;
// a blocked Lock call excludes new readers from acquiring the lock.
type RWMutex struct {
	w           Mutex  // held if there are pending writers
	writerSem   uint32 // semaphore for writers to wait for completing readers
	readerSem   uint32 // semaphore for readers to wait for completing writers
	readerCount int32  // number of pending readers
	readerWait  int32  // number of departing readers
}

const rwmutexMaxReaders = 1 << 30

// RLock locks rw for reading.
func (rw *RWMutex) RLock() {
	if race.Enabled {
		_ = rw.w.state
		race.Disable()
	}
	if atomic.AddInt32(&rw.readerCount, 1) < 0 {
		// A writer is pending, wait for it.
		runtime_Semacquire(&rw.readerSem)
	}
	if race.Enabled {
		race.Enable()
		race.Acquire(unsafe.Pointer(&rw.readerSem))
	}
}

// RUnlock undoes a single RLock call;
// it does not affect other simultaneous readers.
// It is a run-time error if rw is not locked for reading
// on entry to RUnlock.
func (rw *RWMutex) RUnlock() {
	if race.Enabled {
		_ = rw.w.state
		race.ReleaseMerge(unsafe.Pointer(&rw.writerSem))
		race.Disable()
	}
	if r := atomic.AddInt32(&rw.readerCount, -1); r < 0 {
		if r+1 == 0 || r+1 == -rwmutexMaxReaders {
			race.Enable()
			panic("sync: RUnlock of unlocked RWMutex")
		}
		// A writer is pending.
		if atomic.AddInt32(&rw.readerWait, -1) == 0 {
			// The last reader unblocks the writer.
			runtime_Semrelease(&rw.writerSem)
		}
	}
	if race.Enabled {
		race.Enable()
	}
}

// Lock locks rw for writing.
// If the lock is already locked for reading or writing,
// Lock blocks until the lock is available.
func (rw *RWMutex) Lock() {
	if race.Enabled {
		_ = rw.w.state
		race.Disable()
	}
	// First, resolve competition with other writers.
	rw.w.Lock()
	// Announce to readers there is a pending writer.
	r := atomic.AddInt32(&rw.readerCount, -rwmutexMaxReaders) + rwmutexMaxReaders
	// Wait for active readers.
	if r != 0 && atomic.AddInt32(&rw.readerWait, r) != 0 {
		runtime_Semacquire(&rw.writerSem)
	}
	if race.Enabled {
		race.Enable()
		race.Acquire(unsafe.Pointer(&rw.readerSem))
		race.Acquire(unsafe.Pointer(&rw.writerSem))
	}
}

// Unlock unlocks rw for writing. It is a run-time error if rw is
// not locked for writing on entry to Unlock.
//
// As with Mutexes, a locked RWMutex is not associated with a particular
// goroutine. One goroutine may RLock (Lock) an RWMutex and then
// arrange for another goroutine to RUnlock (Unlock) it.
func (rw *RWMutex) Unlock() {
	if race.Enabled {
		_ = rw.w.state
		race.Release(unsafe.Pointer(&rw.readerSem))
		race.Release(unsafe.Pointer(&rw.writerSem))
		race.Disable()
	}

	// Announce to readers there is no active writer.
	r := atomic.AddInt32(&rw.readerCount, rwmutexMaxReaders)
	if r >= rwmutexMaxReaders {
		race.Enable()
		panic("sync: Unlock of unlocked RWMutex")
	}
	// Unblock blocked readers, if any.
	for i := 0; i < int(r); i++ {
		runtime_Semrelease(&rw.readerSem)
	}
	// Allow other writers to proceed.
	rw.w.Unlock()
	if race.Enabled {
		race.Enable()
	}
}

// RLocker returns a Locker interface that implements
// the Lock and Unlock methods by calling rw.RLock and rw.RUnlock.
func (rw *RWMutex) RLocker() Locker {
	return (*rlocker)(rw)
}

type rlocker RWMutex

func (r *rlocker) Lock()   { (*RWMutex)(r).RLock() }
func (r *rlocker) Unlock() { (*RWMutex)(r).RUnlock() }