4 files changed, 30 insertions, 6 deletions
diff --git a/pkg/flipcall/ctrl_futex.go b/pkg/flipcall/ctrl_futex.go
index 5d2ee4018..99410628f 100644
--- a/pkg/flipcall/ctrl_futex.go
+++ b/pkg/flipcall/ctrl_futex.go
@@ -121,7 +121,7 @@ func (ep *Endpoint) ctrlWaitFirst() error {
 	return ep.futexWaitUntilActive()
 }
 
-func (ep *Endpoint) ctrlRoundTrip() error {
+func (ep *Endpoint) ctrlRoundTrip(mayRetainP bool) error {
 	if err := ep.enterFutexWait(); err != nil {
 		return err
 	}
@@ -133,6 +133,9 @@ func (ep *Endpoint) ctrlRoundTrip() error {
 	if err := ep.futexWakePeer(); err != nil {
 		return err
 	}
+	// Since we don't know if the peer Endpoint is in the same process as this
+	// one (in which case it may need our P to run), we allow our P to be
+	// retaken regardless of mayRetainP.
 	return ep.futexWaitUntilActive()
 }
 
diff --git a/pkg/flipcall/flipcall.go b/pkg/flipcall/flipcall.go
index f0e4ff487..88588ba0e 100644
--- a/pkg/flipcall/flipcall.go
+++ b/pkg/flipcall/flipcall.go
@@ -223,6 +223,23 @@ func (ep *Endpoint) RecvFirst() (uint32, error) {
 // * If ep is a client Endpoint, ep.Connect() has previously been called and
 //   returned nil.
 func (ep *Endpoint) SendRecv(dataLen uint32) (uint32, error) {
+	return ep.sendRecv(dataLen, false /* mayRetainP */)
+}
+
+// SendRecvFast is equivalent to SendRecv, but may prevent the caller's runtime
+// P from being released, in which case the calling goroutine continues to
+// count against GOMAXPROCS while waiting for the peer Endpoint to return
+// control to the caller.
+//
+// SendRecvFast is appropriate if the peer Endpoint is expected to consistently
+// return control in a short amount of time (less than ~10ms).
+//
+// Preconditions: As for SendRecv.
+func (ep *Endpoint) SendRecvFast(dataLen uint32) (uint32, error) {
+	return ep.sendRecv(dataLen, true /* mayRetainP */)
+}
+
+func (ep *Endpoint) sendRecv(dataLen uint32, mayRetainP bool) (uint32, error) {
 	if dataLen > ep.dataCap {
 		panic(fmt.Sprintf("attempting to send packet with datagram length %d (maximum %d)", dataLen, ep.dataCap))
 	}
@@ -233,7 +250,7 @@ func (ep *Endpoint) SendRecv(dataLen uint32) (uint32, error) {
 	// they can only shoot themselves in the foot.
 	*ep.dataLen() = dataLen
 	raceBecomeInactive()
-	if err := ep.ctrlRoundTrip(); err != nil {
+	if err := ep.ctrlRoundTrip(mayRetainP); err != nil {
 		return 0, err
 	}
 	raceBecomeActive()
diff --git a/pkg/p9/client.go b/pkg/p9/client.go
index 764f1f970..eb496f02f 100644
--- a/pkg/p9/client.go
+++ b/pkg/p9/client.go
@@ -528,7 +528,7 @@ func (c *Client) sendRecvChannel(t message, r message) error {
 	}
 
 	// Send the request and receive the server's response.
-	rsz, err := ch.send(t)
+	rsz, err := ch.send(t, false /* isServer */)
 	if err != nil {
 		// See above.
 		c.channelsMu.Lock()
diff --git a/pkg/p9/transport_flipcall.go b/pkg/p9/transport_flipcall.go
index 802254a90..69a9f2537 100644
--- a/pkg/p9/transport_flipcall.go
+++ b/pkg/p9/transport_flipcall.go
@@ -85,7 +85,7 @@ func (ch *channel) service(cs *connState) error {
 		}
 		r := cs.handle(m)
 		msgRegistry.put(m)
-		rsz, err = ch.send(r)
+		rsz, err = ch.send(r, true /* isServer */)
 		if err != nil {
 			return err
 		}
@@ -122,7 +122,7 @@ func (ch *channel) Close() error {
 //
 // The return value is the size of the received response. Not that in the
 // server case, this is the size of the next request.
-func (ch *channel) send(m message) (uint32, error) {
+func (ch *channel) send(m message, isServer bool) (uint32, error) {
 	if log.IsLogging(log.Debug) {
 		log.Debugf("send [channel @%p] %s", ch, m.String())
 	}
@@ -162,7 +162,11 @@ func (ch *channel) send(m message) (uint32, error) {
 	}
 
 	// Perform the one-shot communication.
-	return ch.data.SendRecv(ssz)
+	if isServer {
+		return ch.data.SendRecv(ssz)
+	}
+	// RPCs are expected to return quickly rather than block.
+	return ch.data.SendRecvFast(ssz)
 }
 
 // recv decodes a message that exists on the channel.