1 files changed, 37 insertions, 30 deletions
diff --git a/src/pkg/sync/rwmutex.go b/src/pkg/sync/rwmutex.go
index 9248b4b03..cb1a47720 100644
--- a/src/pkg/sync/rwmutex.go
+++ b/src/pkg/sync/rwmutex.go
@@ -4,7 +4,10 @@
 
 package sync
 
-import "sync/atomic"
+import (
+	"runtime"
+	"sync/atomic"
+)
 
 // An RWMutex is a reader/writer mutual exclusion lock.
 // The lock can be held by an arbitrary number of readers
@@ -12,35 +15,22 @@ import "sync/atomic"
 // 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 int32 // number of pending readers
+	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.
-// If the lock is already locked for writing or there is a writer already waiting
-// to release 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 atomic.AddInt32(&rw.readerCount, 1) == 1 {
-		// The first reader locks rw.w, so writers will be blocked
-		// while the readers have the RLock.
-		rw.w.Lock()
+	if atomic.AddInt32(&rw.readerCount, 1) < 0 {
+		// A writer is pending, wait for it.
+		runtime.Semacquire(&rw.readerSem)
 	}
-	rw.r.Unlock()
 }
 
 // RUnlock undoes a single RLock call;
@@ -48,9 +38,12 @@ func (rw *RWMutex) RLock() {
 // 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 {
-		// last reader finished, enable writers
-		rw.w.Unlock()
+	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)
+		}
 	}
 }
 
@@ -61,9 +54,14 @@ func (rw *RWMutex) RUnlock() {
 // a blocked Lock call excludes new readers from acquiring
 // the lock.
 func (rw *RWMutex) Lock() {
-	rw.r.Lock()
+	// First, resolve competition with other writers.
 	rw.w.Lock()
-	rw.r.Unlock()
+	// 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
@@ -72,7 +70,16 @@ func (rw *RWMutex) Lock() {
 // 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() { rw.w.Unlock() }
+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.