Source file src/pkg/sync/rwmutex.go

     1	// Copyright 2009 The Go Authors. All rights reserved.
     2	// Use of this source code is governed by a BSD-style
     3	// license that can be found in the LICENSE file.
     4	
     5	package sync
     6	
     7	import (
     8		"runtime"
     9		"sync/atomic"
    10	)
    11	
    12	// An RWMutex is a reader/writer mutual exclusion lock.
    13	// The lock can be held by an arbitrary number of readers
    14	// or a single writer.
    15	// RWMutexes can be created as part of other
    16	// structures; the zero value for a RWMutex is
    17	// an unlocked mutex.
    18	type RWMutex struct {
    19		w           Mutex  // held if there are pending writers
    20		writerSem   uint32 // semaphore for writers to wait for completing readers
    21		readerSem   uint32 // semaphore for readers to wait for completing writers
    22		readerCount int32  // number of pending readers
    23		readerWait  int32  // number of departing readers
    24	}
    25	
    26	const rwmutexMaxReaders = 1 << 30
    27	
    28	// RLock locks rw for reading.
    29	func (rw *RWMutex) RLock() {
    30		if atomic.AddInt32(&rw.readerCount, 1) < 0 {
    31			// A writer is pending, wait for it.
    32			runtime.Semacquire(&rw.readerSem)
    33		}
    34	}
    35	
    36	// RUnlock undoes a single RLock call;
    37	// it does not affect other simultaneous readers.
    38	// It is a run-time error if rw is not locked for reading
    39	// on entry to RUnlock.
    40	func (rw *RWMutex) RUnlock() {
    41		if atomic.AddInt32(&rw.readerCount, -1) < 0 {
    42			// A writer is pending.
    43			if atomic.AddInt32(&rw.readerWait, -1) == 0 {
    44				// The last reader unblocks the writer.
    45				runtime.Semrelease(&rw.writerSem)
    46			}
    47		}
    48	}
    49	
    50	// Lock locks rw for writing.
    51	// If the lock is already locked for reading or writing,
    52	// Lock blocks until the lock is available.
    53	// To ensure that the lock eventually becomes available,
    54	// a blocked Lock call excludes new readers from acquiring
    55	// the lock.
    56	func (rw *RWMutex) Lock() {
    57		// First, resolve competition with other writers.
    58		rw.w.Lock()
    59		// Announce to readers there is a pending writer.
    60		r := atomic.AddInt32(&rw.readerCount, -rwmutexMaxReaders) + rwmutexMaxReaders
    61		// Wait for active readers.
    62		if r != 0 && atomic.AddInt32(&rw.readerWait, r) != 0 {
    63			runtime.Semacquire(&rw.writerSem)
    64		}
    65	}
    66	
    67	// Unlock unlocks rw for writing.  It is a run-time error if rw is
    68	// not locked for writing on entry to Unlock.
    69	//
    70	// As with Mutexes, a locked RWMutex is not associated with a particular
    71	// goroutine.  One goroutine may RLock (Lock) an RWMutex and then
    72	// arrange for another goroutine to RUnlock (Unlock) it.
    73	func (rw *RWMutex) Unlock() {
    74		// Announce to readers there is no active writer.
    75		r := atomic.AddInt32(&rw.readerCount, rwmutexMaxReaders)
    76		// Unblock blocked readers, if any.
    77		for i := 0; i < int(r); i++ {
    78			runtime.Semrelease(&rw.readerSem)
    79		}
    80		// Allow other writers to proceed.
    81		rw.w.Unlock()
    82	}
    83	
    84	// RLocker returns a Locker interface that implements
    85	// the Lock and Unlock methods by calling rw.RLock and rw.RUnlock.
    86	func (rw *RWMutex) RLocker() Locker {
    87		return (*rlocker)(rw)
    88	}
    89	
    90	type rlocker RWMutex
    91	
    92	func (r *rlocker) Lock()   { (*RWMutex)(r).RLock() }
    93	func (r *rlocker) Unlock() { (*RWMutex)(r).RUnlock() }
The Go Programming Language

Source file src/pkg/sync/rwmutex.go
