diff --git a/src/runtime/rwmutex.go b/src/runtime/rwmutex.go
index bca29d15d0..7eeb559adb 100644
--- a/src/runtime/rwmutex.go
+++ b/src/runtime/rwmutex.go
@@ -31,6 +31,11 @@ const rwmutexMaxReaders = 1 << 30
 
 // rlock locks rw for reading.
 func (rw *rwmutex) rlock() {
+	// The reader must not be allowed to lose its P or else other
+	// things blocking on the lock may consume all of the Ps and
+	// deadlock (issue #20903). Alternatively, we could drop the P
+	// while sleeping.
+	acquirem()
 	if int32(atomic.Xadd(&rw.readerCount, 1)) < 0 {
 		// A writer is pending. Park on the reader queue.
 		systemstack(func() {
@@ -70,11 +75,12 @@ func (rw *rwmutex) runlock() {
 			unlock(&rw.rLock)
 		}
 	}
+	releasem(getg().m)
 }
 
 // lock locks rw for writing.
 func (rw *rwmutex) lock() {
-	// Resolve competition with other writers.
+	// Resolve competition with other writers and stick to our P.
 	lock(&rw.wLock)
 	m := getg().m
 	// Announce that there is a pending writer.
diff --git a/src/runtime/rwmutex_test.go b/src/runtime/rwmutex_test.go
index b78a8e7987..a69eca1511 100644
--- a/src/runtime/rwmutex_test.go
+++ b/src/runtime/rwmutex_test.go
@@ -16,30 +16,29 @@ import (
 	"testing"
 )
 
-func parallelReader(m *RWMutex, clocked, cunlock, cdone chan bool) {
+func parallelReader(m *RWMutex, clocked chan bool, cunlock *uint32, cdone chan bool) {
 	m.RLock()
 	clocked <- true
-	<-cunlock
+	for atomic.LoadUint32(cunlock) == 0 {
+	}
 	m.RUnlock()
 	cdone <- true
 }
 
-func doTestParallelReaders(numReaders, gomaxprocs int) {
-	GOMAXPROCS(gomaxprocs)
+func doTestParallelReaders(numReaders int) {
+	GOMAXPROCS(numReaders + 1)
 	var m RWMutex
-	clocked := make(chan bool)
-	cunlock := make(chan bool)
+	clocked := make(chan bool, numReaders)
+	var cunlock uint32
 	cdone := make(chan bool)
 	for i := 0; i < numReaders; i++ {
-		go parallelReader(&m, clocked, cunlock, cdone)
+		go parallelReader(&m, clocked, &cunlock, cdone)
 	}
 	// Wait for all parallel RLock()s to succeed.
 	for i := 0; i < numReaders; i++ {
 		<-clocked
 	}
-	for i := 0; i < numReaders; i++ {
-		cunlock <- true
-	}
+	atomic.StoreUint32(&cunlock, 1)
 	// Wait for the goroutines to finish.
 	for i := 0; i < numReaders; i++ {
 		<-cdone
@@ -48,9 +47,9 @@ func doTestParallelReaders(numReaders, gomaxprocs int) {
 
 func TestParallelRWMutexReaders(t *testing.T) {
 	defer GOMAXPROCS(GOMAXPROCS(-1))
-	doTestParallelReaders(1, 4)
-	doTestParallelReaders(3, 4)
-	doTestParallelReaders(4, 2)
+	doTestParallelReaders(1)
+	doTestParallelReaders(3)
+	doTestParallelReaders(4)
 }
 
 func reader(rwm *RWMutex, num_iterations int, activity *int32, cdone chan bool) {