sync.RWMutex: rewritten to add support for concurrent readers.

Also made sync.xadd public to help testing sync.RWMutex.

Also added unit tests for sync.RWMutex.

R=rsc
http://codereview.appspot.com/162044

Committer: Russ Cox <rsc@golang.org>

1 files changed, 75 insertions, 0 deletions

diff --git a/src/pkg/sync/rwmutex.go b/src/pkg/sync/rwmutex.go
new file mode 100644
index 000000000..b5e2b55c0
--- /dev/null
+++ b/src/pkg/sync/rwmutex.go
@@ -0,0 +1,75 @@
+// 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
+
+// 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.
+//
+// Writers take priority over Readers: no new RLocks
+// are granted while a blocked Lock call is waiting.
+type RWMutex struct {
+	w		Mutex;	// held if there are pending readers or writers
+	r		Mutex;	// held if the w is being rd
+	readerCount	uint32;	// number of pending readers
+}
+
+// RLock locks rw for reading.
+// If the lock is already locked for writing or there is a writer already waiting
+// to r the lock, RLock blocks until the writer has released the lock.
+func (rw *RWMutex) RLock() {
+	// Use rw.r.Lock() to block granting the RLock if a goroutine
+	// is waiting for its Lock. This is the prevent starvation of W in
+	// this situation:
+	//   A: rw.RLock()  // granted
+	//   W: rw.Lock()   // waiting for rw.w().Lock()
+	//   B: rw.RLock()  // granted
+	//   C: rw.RLock()  // granted
+	//   B: rw.RUnlock()
+	//   ... (new readers come and go indefinitely, W is starving)
+	rw.r.Lock();
+	if xadd(&rw.readerCount, 1) == 1 {
+		// The first reader locks rw.w, so writers will be blocked
+		// while the readers have the RLock.
+		rw.w.Lock()
+	}
+	rw.r.Unlock();
+}
+
+// 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 xadd(&rw.readerCount, -1) == 0 {
+		// last reader finished, enable writers
+		rw.w.Unlock()
+	}
+}
+
+// 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() {
+	rw.r.Lock();
+	rw.w.Lock();
+	rw.r.Unlock();
+}
+
+// Unlock unlocks rw for writing.
+// It is a run-time error if rw is not locked for writing
+// on entry to Unlock.
+//
+// Like for Mutexes,
+// a locked RWMutex is not associated with a particular goroutine.
+// It is allowed for one goroutine to RLock (Lock) an RWMutex and then
+// arrange for another goroutine to RUnlock (Unlock) it.
+func (rw *RWMutex) Unlock()	{ rw.w.Unlock() }