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// 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 (
        "runtime"
        "sync/atomic"
)

// 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.
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 atomic.AddInt32(&rw.readerCount, 1) < 0 {
                // A writer is pending, wait for it.
                runtime.Semacquire(&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 atomic.AddInt32(&rw.readerCount, -1) < 0 {
                // A writer is pending.
                if atomic.AddInt32(&rw.readerWait, -1) == 0 {
                        // The last reader unblocks the writer.
                        runtime.Semrelease(&rw.writerSem)
                }
        }
}

// Lock locks rw for writing.
// If the lock is already locked for reading or writing,
// Lock blocks until the lock is available.
// To ensure that the lock eventually becomes available,
// a blocked Lock call excludes new readers from acquiring
// the lock.
func (rw *RWMutex) Lock() {
        // 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)
        }
}

// 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() {
        // Announce to readers there is no active writer.
        r := atomic.AddInt32(&rw.readerCount, rwmutexMaxReaders)
        // Unblock blocked readers, if any.
        for i := 0; i < int(r); i++ {
                runtime.Semrelease(&rw.readerSem)
        }
        // Allow other writers to proceed.
        rw.w.Unlock()
}

// 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() }